This was the simplest question I got after my talk. However, I didn't know the answer; I simply don't keep track of my play that closely. I probably should have been more prepared for this question, because the last third of my talk was about how we can use Bayesian statistics to model our opponents' strategies, and I used raising rates as an example. In truth, a player's raising rate is (for most players) highly dependent on the situation. The same exact situation never really presents itself twice (at least if we take into account stack sizes and meta-game issues), so it is difficult to gather relevant data. We need to look at similar situations and hope that they are relevant to each other. Poker players do this intuitively: if player A raises way too often in one situation, we guess that he will raise too often generally. I think a very interesting research topic would be to develop a better model of how to generalize from one situation to another, and how to quantify the "similarity" between two different situations. The best analysis I've seen in this regard simply looks at how often players raise given a particular seat position (eg "on the button"), without regard to other relevant factors such as opponents and stack sizes.
An interesting anecdote is that there were a few players at the Bike's 20-40 limit game who thought that I intentionally raised whenever it was their turn to put in the blind bet, and they would call and reraise me more than they would against other players; in turn, I actually did start raising their blinds slightly more often (that is, with slightly weaker hands than I would normally) in order to maximize my EV against their relatively weak calling and raising ranges.
At the talk, I interpreted this question as referring to raising before the flop in no-limit holdem. Even given this rather narrow interpretation of the question, I considered it too broad to give an accurate answer (I didn't venture a guess at the time, but if I had to now, I would say 15%). Instead, I explained two of the most important factors that go into my raising rate. First, the number of players and my position at the table. If I have only one or two players I need to beat, I will raise with way more hands than if I have seven or eight players I need to beat. In the latter scenario, the chances are just too good that someone else has a hand stronger than mine. This position-based thinking is a basic poker concept that should be familiar to any self-respecting poker player. The other factor I mentioned during the talk is the skill level of my opponents. If my opponents are very weak, I will play a lot more hands. Against stronger opponents, many of those hands would probably yield negative EV if I tried raising with them. One thing I forgot to consider in my answer is the possibility of just calling before the flop. I do this so rarely that in my mind I equated "raising" with "not folding," but I do limp sometimes, especially if there are limpers in front of me and my opponents are weak and are unlikely to raise behind me.
In closing, I admitted that I should probably keep better track of how I play. If I played online I would be more likely to keep track, but in live poker it would be very distracting to record all the relevant data.