There has to be enough time for both players to react to the "new" situation. The ball should spot at the end of B's next turn. Here is the rule from onepocket.org, the de facto official rule set, along with some explanation:
9.4 If any owed balls, or balls that have fallen into a neutral pocket are forgotten and later remembered, then instead of being spotted after the current shooter’s inning, they are spotted after the end of the next player’s inning, unless there are no balls left on the table, in which case they are all spotted immediately. In any case, any owed balls are not forgiven, but still must be paid.
In practice, forgotten balls may be spotted at any time after they are remembered, as long as both players agree on the timing; if either player objects to an earlier spotting, then rule 9.4 should be followed.
Please note that playing ‘snooze you lose’ is the rare exception in house rules; it is by no means the standard rule, and it should only be accepted when it is clearly and mutually agreed on by both players before play begins.
