When the latest gym rework originally came out, the diminishing returns for motivation increase on berries would reset after around half an hour. A berry fed half an hour after the first berry fed would give the same motivation increase, and diminishing returns would start again.
Lately I've noticed that all berries fed suffer diminishing returns after the first, and that berries provide less motivation than before. Do diminishing returns reset after a much longer period, or are diminishing returns permanent? I've looked this up here and on reddit, but I haven't found anything recent about berry feeding and diminishing motivation returns.
Best Answer
The timer for diminishing returns appears to be the same as the timer determining whether or not you can feed another berry at all.
From this Silph Road thread:
In the comments, the OP was asked about the diminishing returns, to which they replied: