While some people criticize the idea of trapping against a boxer like fighter who retract his attacks and re-position or immediately follow his previous attack to minimize the chance of time for trapping, who says you have to wait for the fight to start to trap?
There are many pre-emptive forms e.g. in Applegate's training manual, it states the best time to trap is before he moves his attack out. In fact it states clearly that once the attack begins, abandon the trapping and deal with the defense. Spotting the attack before or mid way is quite important in getting a good counter offense, trapping isn't the only alternative, you could attack during this stage or simply get out of the range if you have the time to do so.
While many criticize combat sport fighters for taking too many rounds and it's true to some extent that their techniques do not cripple each other, it's also because they've had too many experience. Skilled boxers could watch a punch like slow mo cinema in a way no untrained people could without getting punched. Thus they take so long to figure it all out.
You could read their shoulders, their hips, their distance in relative to yours, their angle of attack, before they make contact with you. Of course timing is only one response to making trapping work. There's lots of others. The trouble is sometimes schools use it wrongly. E.g. trapping meant for other wing chun practitioners used against boxers. That's recipe for epic fail.