I would caution you to continue to use FOR as it is correct, but I will explain why sometimes you will hear people omit it in your example. The reason is because it's a convention of spoken english. How long have you been waiting for? Answer: 10 minutes. You don't have to answer with the FULL sentence of "I have been waiting for 10 minutes." In the same way they drop the FOR. "How long have you been waiting?" "I have been waiting 10 minutes". It is similar to how people say "wanna" instead of want to. The problem here is that certain quantified time expressions REQUIRE for, such as "I have been studying English FOR 4 years." In summation, for is a cornerstone of perfect continuous tenses and should not be omitted, however, you will hear people do that, especially in discussions about how long they have been WAITING for something, but not with other verbs. I have been fishing 10 minutes. Uh, sounds weird. I have been cooking 2 hours. It doesn't sound right, it should be "cooking for 2 hours, fishing for 10 minutes", but for some reason people omit it with waiting. So just keep using for as you did in the first example and everything should be fine.

Both ways work but it sounds more natural to include "for," so always use "for" if you can remember to.