Depends!

PMK likes lots of agitation, so every 15 seconds.

Most films, every 30 seconds, but Delta 400 in Perceptol seems happy with once per minute.

In trays I'll shuffle negs continuously in pyro developers, but others I may cycle every 30 sec., rotating 90 degrees periodically, and shuffling with one more rotation than the number of sheets per cycle, so that the same sheet isn't always on top.