D&D 5E Downtime Training for Feats?

[Awesome Adam]

30 days should be more than long enough. Otherwise you risk slowing down the campaign or making fears less attractive than ASI.

I believe the context of the discussion was learning feats outside of normal advancement.

30 days to learn a feat in addition to normal advancement would actually make the ASIs a no brainer, where you take all the ASIs and then just learn feats as needed in between adventures.

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In my own campaigns, I'd be okay with the 250 days guideline, since they've yet to ever have 250 days of down time with an appropriate trainer available.
5E's rapid recovery from injury leaves very little reason for the PCs to sit around when they could be adventuring.