I'd personally say that the potential for injury is just as great with a belt than it is without one assuming the lifter knows how to hold his positions (tight abs, mid and lower back). For those that don't have the above down, a belt can certainly help teach them how to maintain a rigid torso, but past that, if the lifter is performing the lift correctly there is no difference.
Injury is more likely to occure when the body moves out of position and this will happen when form is not prioritized or on the heaviest attempts, weaknesses fail -- these weaknesses will more than likely be the same with and without a belt, only difference being that with a belt, weight on the bar will usually be a bit heavier.
To the original post, when to wear a belt is up to you... should you decide to wear a belt. If you want to use one, start now and use it on all of your heaviest attempts. When I used a belt, my general rule was to belt up around 75% - 80% of my heaviest lift done above 85% of my 1RM so that I could get used to the feel. Down sets or volume work were performed beltless.
Most people will get a pretty good carry over with a belt; 20 - 30lbs seems to be the average for squats and deads. Others don't get much carry over.