I don't think there's a definite answer to that. I've never noticed that a villager who is already a master progresses any faster (or slower) than one who has no skills already. Once a villager is an adept they should eventually, given time, become a master. Villagers do differ, though; some learn faster or slower than others, and some seem to like a particular trade more than others, even if it doesn't show up in their 'likes' and 'dislikes'.

It is often a good idea to train several of your villagers up to around adept level in several trades. That way, if something happens to, say your farmer, the village won't starve as one or more others are available to collect food. On the other hand, if you are relying on, say, your builders to complete a particular building while you're away, you might find they've not all stuck to the task and have gone off to conduct research or fish instead.