They run when they have 10 or less health left. Due to their haste it is hard to outrun them at lower levels. If you're trying to kill it with non self healing summons (such as a Fire Devil), watch out because they can retarget very fast.
Try not to let them run away when they are low in hp, as they are fast and can lure you to even more Toads. Low levelmages should avoid them since they are often found in groups and can spam small amount of damage quite fast while reducing your health significantly. Use GFB or other area effecting runes to quickly kill larger groups.
If you're on ground level of the Laguna Islands, mages can run around for about 20 seconds quickly, lure about 20 toads and some other creatures this way and then use GFB, 2 (or even one) shots can be enough for about 1k exp.