Turns out the 100 hour inspection interval myth got me, found the real number in AC 43.13-1B
Everyone in my hangar keeps saying avionics checks are a once a year thing, but I was reading AC 43.13-1B last week and the FAA doesn't actually set a blanket time for avionics, it depends on the install and the operator's manual. My old lead at a shop in Wichita told me 12 months like it was gospel, but the King Air guy I work with now goes by 24 months or 800 hours, whichever comes first, and his unit has been solid for 3 years. So where did the yearly rule actually come from, is it just an ops spec thing or did I miss something in the regs?