If my uncle passed away 5 years ago and hs estate still has not been settled, is there a time limit to how long this can be dragged on?

There is no time period under the laws of all states of this country for how long an estate that is in probate to be concluded. If you want the estate to be concluded in a timely manner you need to speak with the executor and the attorney to see what the status is and what the hold up is.