What do you feel comfortable with? Here are options:
-Raise her rent so she can be month to month. That way she only has to give you 30 days notice. Whatever you feel comfortable with.
-Do a breaklease fee. Maybe 2 months rent fee with one month notice and she's clear?
-Just do a 2 month notice. This gives you time to prepare for another tenant.
It all really depends on your market. Does winter bring rental issues? Where I used to rent places in Michigan, it was horrible. You couldn't rent in the winter. If you hadn't rented your place by November, there was little chance you were renting it before March.