Sale ads can be bumped every 72 hours, that's three days. Just reply to your ad with a Back to the top message.
Also after you have bumped several times you can go thru them and hit DELETE on each to eliminate having a string of "BUMP" posts. Saves space and easier for anyone following the post to get thru it faster. I see posts with 15 "BUMPS" that are useless. Good luck.