You can go with what Magnefine recommends, about 10-15k miles, which would be way overkill under normal circumstances, IMO. I tend to run mine in the 50-100k mile range, but this is with high quality full synthetic ATF in normally driven vehicle with some towing as well (no racing or abuse). Mine looked less contaminated than that one in the pictures, minus the plastic bits from sawing it apart.
My trans builder friend sometimes installs these Magnefine filters after the cooler while the trans is in for a rebuild. They have a flush machine that cleans out the contamination in the coolers after a disintegrating torque converter sends trash downstream. Some coolers are harder to get completely clean, so they either replace the cooler or install the Magnefine after the cooler to catch anything that may yet come out later.
For the average application, I would say it would be reasonable to go 30k miles, up to 50k in some on one of these little Magnefines.