The tijuana board probably has a lot of info on this.  I have found that the crossing is busiest at the regular rush hours on week days.  on weekends it is usually about a 30 min wait between 3 and 7 in the afternoon.  after that their is no wait.  I am talking about walking on foot.   Driving is a whole other story.  I see lines at 3 in the morning sometimes.  It is hard to predict.