That really depends on how you use it. For me, I only use it in the following situations:
1. I need to move the cars around the driveway. This keeps the Prius from starting at all, and I figure that's better than an "on for 15 seconds" burst.
2. If the charge indicator is green when getting off the interstate on the way to work, I'll EV the last half mile, because I know 8 hours from now, the engine can charge the battery while it's warming up at a stop light
3. Same as above, but the last half mile home. Again, only if green.
Like just about anything Prius in nature, if you abuse it, your MPG will drop instead of rise.