If you've got them from classifieds you dont know when they were last serviced, so you should open them up and give them some TLC anyway, IMO. Then you know where you're starting from for the next interval.
I do mine about every two months, but I am pretty religious about cleaning stanchions after a ride, and lubing the seals from the outside everytime too. This lifts the majority of the dirt entrained in the upper part of the seal, and moistens is will oil too. Often the semi-bath stuff won't make it this far through the seal, so it good to do it to be sure.