I sold a mobile home approx 90 days ago and the people who bought it have still not moved it I was wondering how long they had by law to move it off my property

I am not aware of any law that says it must be moved. So far you have allowed it to be parked on your property so that's totally legal so far. If the title has cleared and the deal is closed you can notify them that they must remove the trailer from your property. The time limit you must give varies from stste to state so check locally.