Its funny to see this now, as this very topic of conversation came up for me just last night...
I have always been a switch, although it took me forever to admit to my submissive needs, and even longer to my Dominant ones. It took the love and support of a very special person who had the strength and patience that allowed me to grow.
I agree with you 100% about switching "on the fly" I find it exceptionally difficult on an emotional level to be Dominant one moment and submissive the next. I dont have a time frame per se, but if I am submissive, then thats the role I stay in, and vice versa...
Having said that I havnt found it difficult meeting or having my needs met. I think with both parties working together and with open communication the relationship between both switches can work very well. Id like to believe if you have the love and support of a good partner, those times when the need within you conflicts are very rare and easily resolved.