For a sub box it's usually between 1/4", or maybe 3/8". 1/4" fiberglass is actually stronger than 3/4" MDF, just make sure you make the box nice and sturdy, don't skimp or it'll resonate like crazy.
depends on the box type and style.. usually around 9 to 13 layers so it won't flex too much and be sturdy enough to hold the subs without cracking etc.. reinforcing it with wood underneat quite a bit helps in that too don't rely on the glass alone to support weight.