You typically "main" one class each raid tier and the reason is because you have got the best gear from the raids and from weekly capped tomestones for that class. Since some classes share gear with other classes, some people main all of the classes within that role (such as the healer role, the tank role or the ranged physical role).

You can still play all of the other classes in raids and be fine with crafted gear, it just isn't your main because you haven't been using your weekly obtained gear on it. When raids have been out for some time, you may have got all of the gear and be looking for another class to get gear for but you are really "maining" one at a time.

For how long it takes to get crafted gear for secondary classes, you just go up to the market board and buy it.