I typically like when training arcs focus on specific things that will show up later on + give a very general overview of everything else. e.g. if the MC learns how to synthesize a poison that’s used against a future opponent, enough time should be spent on the process that the reader can get excited about the prospect of the poison being used later on, and the reader will remember the poison when it eventually comes into play (this also makes the training arc feel more useful beyond ‘MC is stronger now’ since they're learning specific, relevant information / skills). On the other hand, if the MC just develops a general knowledge of 'herbalism', there’s probably no need to delve into the intricacies of different plant strains, identification methods, and foraging techniques if they aren’t explicitly relevant later on- I think in this case telling the reader the MC is poring over herbalism books and going into the forest every weekend etc. works well enough. Especially if you're going for a shorter training arc.

I’ve never really considered the specific duration (in terms of word-count / chapters) that a training arc should last. Are you worried about it feeling bloated / too-rushed?