I am, it seems, a very lucky freelancer in that I very, very rarely suggest ideas, except when I think a story is wonderful and fascinating and ought to be told, or if it's topical and I think it will be fun to write. I am also fortunate in that I have been in the magazine business for so long that I know lots of the people who commission me so can call or email them informally. However, I also have lots of experience of commissioning freelancers when I sat on the other side of the desk as a staffer and do understand what is going on in their heads most of the time. When I offer a story to someone it is usually very specific to the magazine and so not directly tranferable. However, if I really wasn't getting a response, I think it would be perfectly fair to tweak the idea and submit it to another magazine. However, I would be careful about submitting to a direct competitor. IME magazines do not like 'their' writers appearing in the competition - ie you do not usually get the same writers on Eve and Red, say. It is better for your career, IMO, to become a pet freelancer for a select group of clients/magazines which don't clash with each other, as this will mean regular work for you. As for comm eds who don't commission, well, they do the donkey work of commissioning, but on glossies they normally wait until they can get the approval of the editor, either immediately or at the monthly features meeting when the lineup for the next issue is finalised. It costs a lot of money for a magazine to have a lot of 'stock' - features commissioned just because they were interesting but which haven't found a home in a particular issues. The cost of 'stock' is one of the reasons, I understand, why Real magazine recently folded. So mags tend to avoid this situation. So if they love your idea, but can't find an issue where it works - perhaps because the 'slot' - ie real life/emotional/sex feature - is already filled, the commissioning editor might but it to one side and resubmit at the next meeting. However, it is my experience that editors (who are fickle beings!) can get the idea that a feature is 'tired' just because they have heard about it before and unless the features ed gives it a real hard sell it is unlikely to be commissioned.
So, to answer your question, how long should you wait....it depends! If the editor is really keen but is waiting for a features meeting, wait until then and then try your luck elsewhere. Personally, I would probably say to the comm ed that if they are planning to go with the idea then you won't approach anyone else with it, but if not, then would it be OK to try it elsewhere. If they just sit on it, can't give you a yes or no and you feel time is running out for this story or you just want to get on, then try submitting it to another magazine (but tweaked to suit that particular publication) and if they say yes, then that's fine IMO. It is tricky, I won't deny it.