Funny thing, I just updated my driver. I've been swinging a Ping i15. I wasn't having any problems hitting it down the fairway. My yardage was in the 270ish range and ball flight was good. However, after recently changing out my 3 wood - another Ping i15 - I found that the shaft and overall swing weight was heavier than current models. What I previously fitted into was due to the technology available at the time. In order to produce lower spin and trajectory, the earlier recommendation was heavier shafts. With newer technology in the club face, all that has changed. So, I decided to give my driver a comparison test. As with my 3 wood, my swing speed was 5 mph slower compared to newer drivers - launch and carry were also both lower - ball speed increased. I walked out with a black R1. : )
To answer your question, "when do you upgrade your driver?"... When you can prove the technology in your current model is lagging behind newer models. For me, proof is in the numbers. If you're open to other manufacturers, I suggest taking your driver into a local shop where you can compare the performance of several offerings. If there is no marked improvement, then you know what to do.
Good luck and enjoy.