You set it in your slicer program. In Simplify3d it's under "Edit Process Settings"->Temperature->"Heated Bed Platform"
You can have it change by layer if you want.
For ABS, I use 110 C for the first 10 layers, then 100 C for the next 10 layers, then 90 C til the end.
Since I have a glass bed, the part will pop right off when the bed gets below 40-50 C or so.