Mac Pro average lifetime?
Well, the Mac Pro went through quite a long period of stagnancy recently, which is likely the cause of the disparity you see between the iMac and Mac Pro in terms of software compatibility.

According to the MacRumors Buyer's Guide, the Mac Pro has by far the largest average time between refreshes (449 days), so new models are sparse.

Because of that, it's a good time to buy, if that's what you're asking. Though there could be a minor spec bump or price drop at any time (those are more or less unpredictable from afar), it's not worth the risk of waiting for one. The December 2013 update is likely the only major refresh we'll see for quite a while.

As far as average lifetime goes, the Early 2008 Mac Pro shipped with OS X Leopard (10.5), and is still supported in Yosemite (10.10), so it has received five software updates so far (and there could still be more). 

Based on that, you could expect software compatibility to last for upwards of 6 years on a Mac Pro bought today, if that's how you define lifetime. However, the Mac will of course function on old software for much longer than that.