There is no time limit for each lift or for the length of the session in which they are all performed, but they must all be performed during one session—i.e., you cannot leave the area to rest or perform other activities between the three lifts.
Anyone in a position to attempt a legitimate CrossFit Total should be familiar enough with their capabilities on the lifts to have a fairly good idea of just what might be possible for a one-rep max (1RM). This number is what you warm up intending to do. A meet situation will involve three attempts, and this is a good way to determine a true 1RM.
The first attempt would be a weight you know you can do for a heavy set of three. The second attempt would be a weight you know without any doubt that you could do for a single, having just done the first attempt. And the third attempt is the weight you want to do, based on your performance on the previous two attempts.