Many growers test the germination rate after harvest, and good growers will often sell seed in ounces or pounds "PLS," or pure live seed. If 80% of the seed is good, and you buy 10 ounces PLS, then the package of seed will actually weigh 12 ounces. After many years of collecting, they get pretty good at getting the seed at just the right time. Low germination rates hurt them on shipping and handling costs.

The advantages of collecting your own seed are many. It is inexpensive (in cash, if not in time). It encourages you to learn more about the plants. If you collect from wild areas, you may be saving unique genes or traits from your region.

If you collect your own seeds, I highly recommend that you follow good seed collecting ettiquette. Never collect more than 1/3 of the seed from any species in any spot. And always get permission fromthe landowner (on private land) or the land manager (on public land).

Purchased seeds are not necessarily better in quality, and I think collecting is a lot better way to get to know local plants and to make your garden a reflection of the local ecosystem. One thing seed merchants have that you don't is lots of experience collecting wild seeds, so they know when to collect each species. You have to figure out when to collect by watching and a little guesswork, but it sounds like you're doing pretty well. nobody gets 100% of the seeds he/she wants on the first try, and I think trial and error is part of the fun (well, maybe not the error part).

When is the best time to sew seeds I've collected ?

Probably November, grass can be sown in the spring I think as can stratified seed of others.