How much HD space should I allocate to Time Machine

I have a 160 gb HD. 30 gb is allocated to Parallels which I do not backup via Time Machine. Assuming at some point I fill the remaining 130 gb with data, how much space would you recommend allocating on a 500 gb HD for Time Machine?

I want to partition the drive so that a portion is free for other uses. My goal is to minimize my troubles in the event of an internal HD failure. I don't really care to be able to revert my machine back to the way it was a year ago. A couple months worth of history would suffice.

Any ideas on a formula to use?

Since you're essentially backing up potentially 130GB of data, I would say double that amount would be good, so about half of the 500GB external.