Oh it takes foreverrrrrrr.

Cerriscapades on Youtube has several videos where he documents the progress of his succulents from seed. From what I've seen it seems like even with a whole year, you might not end up with plants big enough to fit a 4'' pot (maybe 2'', but even then it varies). You'll get faster success with propagation, though growing from seed can be fun if you want to experiment with hybrids. Just as there are different propagation speeds for different people, there are also different seed growing rates. I have several props of the same species that, depending on the environment and how much I fertilize and the amount of pot space they have, they grow at very different rates.

You said succulent and not necessarily cactus. My recommendation for a really rewarding grow hands down are plumeria and adeniums ... more so if you are in a tropical, semi tropical climate. If you want to try 'fat' or plants with a caudex, go for Adenium arabicum. They are cute little fatties and bloom in 6 - 16 months in the right grow culture.

Dino_Diamond King Arabicum · More Info

A. Arabicum "KO_Brazilian" · More Info