I was asked this question in an executive management meeting yesterday. The answer is: Immediately, regardless of experience!
Body language indicated that this answer was not believed by at least 7 of the 11 managers present. I explained that when promoting or hiring you should always do it on a trial basis. The manager and the employee in the new role should have the veto power to call the whole thing off within the first 90 days. You should establish a core score or core scores that will be reviewed daily and weekly in order to insure short and long term success. If the move to the new position or task is not successful then a new position or task will be found quickly.
I then asked how many of these executive managers followed this process? One out of eleven raised their hand and the other ten agreed. They also agreed that this manager had the best managed and most engaged division in the entire company!