If a concert is general admission, and day it starts at 8, how early would be a good time?

Its standing room only, unless you pay $100+ for a chair. Depending on where you want to stand, I would get there an hour early.

I would say 6 or 6:30. Depending on sellout or not

There should be somewhere posted either on the event site or on the Fillmore site that will say something like "doors 7. Show 8" then you can decide when you want to go.

630 is what I would do NO later than 7 I've noticed there multiple times they tend to start a bit earlier than advertised.... hope this helps

I would say it heavily depends on how popular the show you're going to is. If it's sold out, I'd say at least an hour before doors are open, maybe even more.

If you want to be upfront, then an hour plus. If you want to be up 30 would work

1 hour early is my recommendation for that venue, but it really depends on the show.