An old rule of thumb is a 35 mm film canister will hold about 1000 seeds. You'll get about half canister per oz. in moderately seeded ounce of bud,or roughly 5-7 grams of seed. Heavily seeded bud can fill the canister per oz. of bud. Do the math when you know your yield of seeded dry bud.
As for how long it takes for a naturally pollinated female to produce mature seed. It varies by strain,but generally it's 3-6 weeks after pollination of the female flowers. Sometimes it can be a week shorter on the low side and couple of weeks longer on the high side.When 60-70% of the seed is mature,so is the bud,it's at peak THC production. If you want a stonier high you can let go a couple of more weeks to let the other cannabinols develop.
How do you know when the seeds are mature? The calyxes holding them begin to split,and you can see the seed. Pull a couple out,check for firmness and color. A firm seed,tan to dark brown in color is what you're looking for.
Indicas are notorious for producing a boatload of seeds. A healthy outdoor grown Indica,fully pollinated, can produce literally thousands of seeds.