I used a brooder plate last year for the first time. The chicks spend less time under it than you would expect, especially if you are used to a heat lamp. They DO use it though, so don't take it out.
Something to consider: are you tilting one side up higher than the other, and raising it a notch or so every couple of days? You want the lowest side to be just touching the back of the smallest chick when laying down, and you want the talles side to be touching the head of the largest chick when standing up. This will allow every chick to find it's best zone.
The chicks run under, get warm, then come back out to play, just as they would do with a mother hen. I found my plate-raised chicks feathered out a lot faster than my lamp-raised chicks, and were more hardy and prepared for temp fluctuations when I put them outside.
I took my brooder plate out at 4-5 weeks, as my chicks were all well feathered at that time. I kept them inside the brooder in the basement a couple more weeks, where it was temp controlled. They were outside by 8 weeks. (your situation will vary based on your location)