I wouldn't extend him right now.
I suspect an extension would be something like 2 years, $12M - $14M (Carlos V got 2 yr - $10M from Cubs); but lets be honest, he hasn't been able to "stick" in the rotation during his career and at 31, it's doubtful that will change (he's just on a hot streak). At $6-7M/year - he has to be in your rotation to give you value because he'd have to be a shut down setup man to give value at $6M/year in the pen.
He's a useful, versatile pitcher - he's just not someone you want to sink a lot of $ into.