how often to change wheel bearings

Only when you need to. I got more than a 100,000 miles per set on my Forester but I live in the mountains which is hard on wheel bearings,

After replacing a couple hundred of these bearing/hub assemblies what generally develops is that the right rear goes first, perhaps followed within a year by the left rear. Fronts tend to be more durable, with no established side preference. So replacing rears as a PAIR is perhaps not imprudent if one lives far afield from a wrench. Fronts should NEVER be replaced as a pair, as their wear correlation is weak. $200/corner gets it done...nothing tricky nor special except to not damage the ABS sensor.

I ended up replacing the fronts and rears on my 2003 Forester in pairs as they went out at the same time. Both sets were replaced at nearly the same time. Can't remember which set was first but replaced at around 100k but at 200k they were still going strong.

Jeez - I had to do both rears at 60,000!! At $650 per side at my Subaru dealership!! Is this 2015 Forester an average Subaru??