I did a quick search on this for you and found this:
http://www.homebrewtalk.com/f30/how-long-do-you-keep-fruit-secondary-193742/
Even though not a whole lot of people chimed in, the majority didn't leave it past 3 weeks.
If it were me, I'd probably transfer and let it sit in tertiary to avoid any fruit decay issues. Besides, if you come back and taste it and the cherry flavor isn't where you like, you could always rack again onto more cherries!
Anyone else have an opinion?