That's a BIG question. The answer is that seeds of some species germinate very quickly because they need to produce strong plantlets before winter comes in cold climates, some take three cold winters, some require extreme heat (fire), some need to pass through the gut of birds to strip away inhibitors in the flesh of the berries, some require light (as in species where the seed does not want to germinate when covered in snow), some have tough protective shells that disintegrate at varying rates depending on the weather, so that they germinate over a long period, maximising the chance that some seeds will survive and develop, etc. etc. They have alle volved different survival strategies according to their local environment.
Gold1locks is quite right, it varies enormously and for all of those reasons.
Tomato seeds can be extremely quick if they are given the right conditions. This year I had some germinate within 3 days!
The longest I can remember were some Japanese Acer seeds, which took at least 3 years. They were in a pot I stuck in a corner of the garden and forgot about until one day I noticed a tiny unusual-looking shoot. It's now a fine 6ft 'Autumn leaved' type tree, so worth the 10-or-so years wait!
I'm with Bob, tomato seeds - feisty little blighters!
Cosmos definitely! My cosmos purity popped up in two days.
Rocket (if the temperature is right)
If you put Dianthus seeds in a petrie dish and put them in the salad compartment of the fridge, they will germinate overnight.
Another vote for cosmos