Balls remain on the List for one year. Random tests are carried out by The R&A and USGA on selected balls to ensure that the ball characteristics remain the same. If a conformance issue is identified during random testing, the ball may be removed from the list.
Normally, only the Committees in charge of competitions involving expert players (professional-level players and/or players at the highest levels of amateur golf) should introduce a Model Local Rule requiring a player to use a ball included in the List of Conforming Golf Balls, the penalty for breach of the Local Rule being disqualification. However, if this Local Rule is not in place, then the assumption is that a ball used by a player conforms unless there is strong evidence to the contrary e.g. a player used a 1.62 inch diameter golf ball (a ball which used to be in circulation and was permitted under the Rules before finally being prohibited worldwide in 1990).