I usually get my regs serviced every other year and always do a test dive with them after being serviced before any trips because there is a better chance of them failing right after service than 13 months down the road. Regs that are only used once a year should be checked more often because they may have corroded from not being put away properly or the valve stuck to the seat.
Disclaimer, I dunk wash my regs while pressurized and then flush the 1st stages with a squeeze bulb to get all of the salt out after every dive even on liveaboards.
I service my BC’s myself, once before a trip I told my daughter to check her equipment, she didn’t and her BC big hose had more holes than Swiss cheese and the dump valve cable had snapped. BTW, if you ever need that cable, some manufactures won’t sell it to you go and / or it only comes with the dump valve. Go to West Marine, they sell SS cable by the foot, have the ferules and the swedge press right there on the bench to press them :)