If I have tor running for a while, how often will my "identity" change?
Say I don't ever click the "New Identity" button, but just have the regular browser bundle running for some days. I'm guessing my identity will change since nodes are not always up, but does Tor enforce creating new routes at some interval? (If so, how often?)
but doesn't it keep the cookies you got until you click the "New Identity" Button?
First of all, Tor builds new circuits regularly - that has been covered in For how long does a circuit stay alive?.
However, the New-Identity button in the Tor Browser Bundle does more than just rotate to a new, unused circuit. It also tries to clear cookies and maybe more in order to make your future requests unlinkable to your previous web visits.
That property you don't get at all just by waiting for some time.