Annually for routine physical exam, rabies and any other booster shots.
As required (usually about every 6 months?) for blood work related to ongoing meds she takes. They have to check her blood levels to make sure she is keeping the effective level in her system... and over time that can change due to changes in her metabolism, so they test to be sure she is maintaining the proper amounts.
And also, if she seems ill or she is injured. This has been a rarity, but she did develop a limp once that seemed to come and go, and turned out she had a tick borne illness that was easily treated with antibiotics.
Remember, animals' life spans are shorter than ours. So a year in between vet visits, is akin to several years between doctor visits for a human.... just to put it in perspective.
As pets get up in years (over 10?) the vets encourage you to bring them in 2 times per year. I understand why, but it certainly doesn't hurt their bottom line, either. My dog turns 10 in a few months. Since she is already seen several times a year for other things, I probably won't be increasing her number of visits just due to aging. As needed (and that means rabies shots, b/c those ARE a need; we live in an area with lots of wildlife).