>How much time can a show spend on build-up before you demand the plot move forward?
Depends on the show and what it's doing.
>How many episodes can the main character be absent before you need things to get back on track?
Since I self-insert into the main character, that's actually a problem.

What would you refer to as build-up? More episodic stuff that doesn't clearly advance things?
As for the MC, in principle I don't care. I care if a good character I like stops appearing, but them being the main one doesn't confer them a special status.

If it's really good then it doesn't need to move forward at all. The exception is filler material, but I don't think that happens much these days. But, I think it needs to be clear about what the show will entail early on, and not suddenly pivot for episode or two in a completely different "genre". I think it's better for a dramatic show to have a fun episode than a fun show to have a dramatic episode, especially if it goes into politics and stuff.
Story progression though? As long if it's still generally heading in the same direction I don't care if it takes a while if the characters are good.

All of them. Never move the plot forwards and have the characters move on from school. Keep them there forever and the good times continue forever.

>>119717
Episode 1:
We are informed that aliens are attacking Earth. Humanity scrambles to bring together a team of high school students to combat this threat.

Episode 15:
So far, every episode has brought news of people dying, massacred by the vile invaders from outer space. The protagonists are very disturbed by it and discuss it with great vigor during lunch on the school roof.

Episode 52 (last episode):
One of the characters confesses her love for another, but the declaration ends up inaudible over the noise of heavy bombardments. "Eh? Nandatte?"
To the background of the burning city, the girl suppresses a tear, bears a slightly bitter smile, and says: "Eh, it's nothing."

>>119719
I want 10, no, 20 seasons!

>>119688
In the OP show, they spent five episodes introducing characters and are just now having the different factions start to interact meaningfully. It's still been interesting, but I am wondering how much they're going to be able to fit in the remaining time. It's not episodic or SoL at all, but at the same time not much is actually happening while they set the stage.

It's different from, say, Code Geass or Railgun where they periodically set the important stuff aside to get into school shenanigans. That can be a lot more frustrating if it isn't used to build character motivations or relationships and just ends up feeling like a filler episode.