The longest I've every kept a mantis is about a year, though I would guess that 15 months is possible with some species if you keep them really cold and feed them very infrequently.
Typically, females of popular species will live upwards of nine months if you don't powerfeed them and keep them warm.
I think nine months is pretty good when you consider how short warm season is in most parts of the US and the short opportunity we have to appreciate our natives. However, native mantises that are born and raised in captivity will live just as long as exotics.