Timer in XCode?

If I read the docs for NSWorkspace correctly, you should be able to receive notifications in any app about what other apps are activating or deactivating. If I am correct, you could write a simple app that keeps track of how long a given application (e.g., XCode) is active (as in foreground) and present you with some sort of summary log that you could decipher. Of course, if you have to go spend half an hour in the "reading room" and leave XCode active, that might skew the numbers, unless you have auto-sleep and are getting notifications for that as well.

 

That reminds me of a small app I wrote for a friend years ago (screenshot, download).

This doesn't deal with sleeping (my friend didn't need that, and I was lazy), and it won't help you with discerning between individual Xcode projects, which is probably a deal breaker for the OP. But if a very simple apps monitor is all you need, then it might do the trick.

 

What about an Applescript?