*peers around looking for a Cati - nope, not one here*
Seeing as I'm not online, I can't just look up the help file on this to help me answer how long the delay should be. The response would be dependant on how trample is actually viewed (ie: what the help file says about it).
If you consider trample more of a charge/trample then it would take time to charge/trample/run over victim, but as you can continue fighting in melee when you trample, I'm assuming it's not being used this way.
That gives us a horse kick (which would be different than kick I guess as centaurs don't always have kick), which if you were in melle, you should be able to fight with your weapon and kick out with your hooves during the same round.
But, if you're thinking of the rearing kick, then there should be a delay - as you (or your mount) would have to turn around to kick, and I would think that you really wouldn't be able to use your weapon during that round.
so, my 2 cents. tear apart at will.