For how long should I keep my protagonist alone in my post-apocalyptic story? How should I go about it?

Ok fellow people. I'm trying to write this hard realistic horror-action-adventure abomination of a story. The plot centers around a city dominated by giant creatures that seemingly popped out of nowhere and have killed everything alive on the planet, from bacteria to elephants, the only thing alive and breathing (for now) is my protagonist and those creatures.

So you may understand that the protagonist is completely alone, he awakens by himself in a place he doesn't remember, and everyone he manages to find is dead. 
My objective keeping him isolated from any interaction is so that these extreme life or death situations sharpen and roughen the protagonist. All his life, he has been growing up in this comfortable, loving, soft environment that hasn't really challenged him in any way. And although he isn't a bad person, he is still very flawed, and he is going to have to fix himself fast if he wants to get out of that place alive. 

I do not know why, but all my story prototypes seem to revolve around characters that are by themselves, which I think is a good way to get big introspection onto his mind and narrative. While also managing to gather some character development and reveal progressively the factors that made him as he is.

I have already written about 4 chapters and have a good rhythm, the only problem is the fact that I do not know how much should I keep this up without introducing other characters. I do understand that some people may dislike the idea of my protagonist is a weak, cowardly, indecisive piece of garbage from the start. However, I feel like there is much joy in growing as a person the way I'm setting it.

So although you can't read much of my novel (it's only one chapter released, and 3 on schedule), I would appreciate it if some of you could give me tips to keep both the character development ongoing and the reader's interest up. Or just any recommendations about introducing new characters to the play without slowing the rate at which the protagonist grows.

Thanks for reading. 

Re: For how long should I keep my protagonist alone in my post-apocalyptic story? How should I go about it?

I say keep it up as long as possible. Sure, some people might find the lack of dialogue boring, but that's a them problem. If you find yourself writing a number of stories where the MC is completely alone, that means this topic interests you. As it interests you, I'd bet you write it well.
As far as reader interest goes, just make sure there are always stakes. The MC will have to find food, water and shelter. And then upgrade defenses of that shelter. And then continue going out to find more water and food, while avoiding monsters. And maybe a particularly vindictive monster moves into the neighborhood, making it impossible for MC to leave the shelter. After that, the supply of soup cans at the local wal-mart is running low, so MC will need a long-term food plan. Possibly involving gardening. What's the weather like; is that a massive garden-destroying storm on the horizon?
As long as the MC always has a goal and something to stop that goal from happening, you can keep the readers interested.

Re: For how long should I keep my protagonist alone in my post-apocalyptic story? How should I go about it?

I get bored by characters on their own, so I am not in the target audience for a solo survivor story.

However, for storytelling in general, the way to keep things interesting is with goals and conflict. These don't have to be things like fighting an army or defeating a dark lord, it could be something as simple as 'find a water source' and the conflict could be something like trying to find a generator to hook up to activate the plumbing, or a river frequented by hungry beasts.

When a protagonist has a goal to work toward and obstacles getting in the way of that goal, things tend to be more interesting than if he were wandering about freely without purpose or opposition.

Re: For how long should I keep my protagonist alone in my post-apocalyptic story? How should I go about it?

Fans of that genre usually prefer solo main characters. But yes, it's good to have other people around. As a compromise, I think just have characters who don't stay around for long. You can kill them off, maybe they'll betray the mc, maybe they have different goals and separate. Essentially, just have short "mini-arcs" where there are other people. Meeting a group of survivors who quickly dies after for plot reasons is a good introductory group I say. 

Re: For how long should I keep my protagonist alone in my post-apocalyptic story? How should I go about it?

I'll throw my two cents in. Stories that only have one character for too long aren't my jam. Which is totally fine, not everything has to be my jam, but I'll try to explain why. 

Defiance of the Fall is a popular read on here that I enjoyed a lot, and I think it struggled with the same thing you talking about. A guy wakes up in the wilderness with a hatchet after the world has changed. Like you said, its interesting to see the character struggle with surviving and figuring things out on their own. But it went on and on. The worst part was that once the author got to the characters they were pretty interesting.

I think some of this might be able to be mitigated by having some form of non-verbal character show up. The dog from I Am Legend or the cockroach from Wall-E come to mind. Just something where I'm not stuck inside the characters head for five hundred paragraphs.

But like I said, Defiance of the Fall did this and I still read the whole damn thing, so who knows :)  

TLDR: 5-10 short chapters before I start wondering where the other characters are.

Re: For how long should I keep my protagonist alone in my post-apocalyptic story? How should I go about it?

That's a real serious question!

And the answer is entirely dependent on your target audience.

If you want to be somewhat realistic and appeal to an audience that searching that flavor in a post-apo story, survival alone is nigh impossible.

If you want to keep that trope of solo character surviving ludicrously challenging situation, there is a really BIG audience for it too. And I assure you: a lot of person love the zero to hero progression litrpg ;)

There is also way to introduce other characters but remain mainly "solo" if you want to:
-crazed hostiles human survivors.
-the MC being rejected for whatever reason (weak at the start, crazy in the middle, frightening close to the end?)
-the MC and others characters goals not aligning.
-He might also interact with the dead like in most necromancer story, giving some interactions and back-story without making those permanent characters.
-You might also introduce side character just to brutally murder them three chapters later