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 Term of Competition requiring a player to use a ball included in the List of Conforming Golf Balls, the penalty for breach of the Term being disqualification. However, if this Term 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).