Is it an adult? Garters have higher metabolism than many other snakes. What I would do is feed the garter every 5 days A good size meal when the other's eat and a snack mid way It would probably be ok every 10 days, but it's really not too tough to offer it something midway between feeding the others. If it's a baby it'll need to eat even more often. As you know, snakes can go long periods without eating and be fine, but I would have this be the exception and not the norm