If you're looking for that info on racing or acro quadcopters, a good starting point would be to look at what types of tilted camera mounts people are using. Just as Neil said, it can vary by a LOT depending on the type of flying. The tilt determines how fast you're flying in that direction, so it's anything between anything over zero, and can surpass 90 for a bit if you're losing altitude. The thrust to weight ratio will determine how close you can get to 90 degrees before losing altitude. 22.5 and 30-degree camera mounts are common for acro/racing, so that should be somewhere around average angle during that type of flying.