If you are playing single player, you can take as much time as you like. The progression depends on what you do, not how long it takes you to do it, and most of it is based around which bosses you have beaten so far. Usually the bosses grant some prize that allows you to progress to the next tier, like you beating the deer granted the antler needed to make the pick so you can enter the bronze age.

Multiplayer is a little different, that doesn't track individual progress so the pace is generally set by whoever is the most advanced at the time. If you are still struggling through bronze age while a friend on your server is fighting the (current) final boss, you may get raids of critters beyond what you are currently equipped to handle. Even on MP though, the bosses aren't harder if you are fighting them alone, though they do get adjusted a bit if you are fighting alongside others on the server.