Is that 5 minutes for an entire session or five minutes working on a single behavior? My batsharks will happily take turns, practice staying while the other works, and switch activities every few minutes for hours on end. They also offer behaviors constantly and hang out on their perches just in case I feel like giving them a cookie.
A lot of dogs will happily go all day, but there can be a decrease in clarity on part of both the dog and the handler, and relying on long sessions to communicate a behaviour can become a crutch at worst, and is simply unnecessary at best. Again, I'm specifically thinking about sessions where you want to accomplish 'X' expeditiously -- longer =/= faster.