Shes 1000, its in the lyrics in one of the songs on adventure time she says" Im not mean i'm a 1000 years old, and i just lost track of my moral code"
p.s technically shes 1001 cause that show started like a year ago and that was one of the eairlier episodes. or she could be 1002 if the show is 2 years old im not sure but hope this helps