There is usually a wear indicator that will let you know. Starts making noise.
You can measure the thickness of the pads if it's less than 3/32nds it's going to be getting close.
If you do the pads early enough you won't have to surface the rotors. If you let them go and the rotors have grooves worn into them from the pads then you'll have to resurface or buy new ones.
You can only resurface rotors so thin before they have to be replaced. I think Tacoma rotors are 22mm thick and you can shave 2mm before they become unsafe to use. So no less than 20mm.