You could, I suppose, but it's a little awkward. If you want to make it a yes/no question, you could say:
-I will be in the shower.
Actually, now that I think about it, it's not that awkward, because the "for" is kind of assumed when you say "Long?" So yes, you can do that, but you have to be careful about how you use that construction.
Also, I would say that you can definitely do it with "often." Here's an example:
-I like to go shopping.
It implies that the second person is saying "Do you like to go shopping often?"