I've been looking into this problem for a while now and here are the few things that I came up with(however NaN of them feel quite right):

1. Test code coverage
2. Amount of bugs / Amount of features (%)
3. AVG Page load time (assuming we talking about some sort of website)
4. Cyclomatic complexity (spaghettines of the code) should be small
5. Essential Complexity (unnecessary loops and ifs)-should be small

So the question is how do you guys do it? and at what point (every 2weeks/ on the time of release/ every 1/2 year /every day/ every commit/ once in a blue moon)