I generally require training for the first level of every class that the character takes, with training assumed for level 1. A PC hoping to multiclass can begin training before they actually have enough XP to reach the next level, and they can train with the appropriate party member during downtime each day, which allows them to level up simultaneously with everyone else unless they are multiclassing into a class to which the party has no access in the middle of the wilderness. But at that point, it seems implausible for the barbarian gain wizardly knowledge out of the blue anyway. For PrCs, the character usually has to learn from an organisation or a previous member as well (although PCs with access to memory stones have been able to circumvent this when there was no NPC handy). If they want to advance in a class to which they already belong, I let them train themselves and assume that they do so during watch hours or other downtime, unless they specifically tell me "I choose not to train at all, so I'll have to spend some time to level up when everyone else already has their level." For obvious reasons, this doesn't really come up much.