The sooner you can treat it the better.   Might want to add a little food coloring if you have a lot of stumps in one area...this will help you see which one you have treated easily and or at a distance.   I use tordon mostly and it's already a purple/blue type color.   I have heard stories of tordon leaching in the soil however and killing other things...   I have not had that issue personally.   I have also used straight gly before.   Just put some in a container and brush it on or put in a squirt type bottle and cover the entire stump face.   I have also used a cordless drill to put a cavity in the stump as well...   Lots of ways to do it.