Shoptask cnc steppers

I've got a question for the group. I've go a shoptask 3 in1
I'm trying to set the maximum step rate on the motors. The question
is how much force should the X/Y table apply before losing steps
I realize that it will vary depending on what material is going to being cut
I'm just looking for an approximate figure I'd like to get the most speed I
can and still be reliable. Any ideas