brumation advice

I have a mature tricolor milk snake (?Ruthven's kingsnake). I live near San Francisco. I am wondering if I should move my snake to the garage for 3 months during the winter (temps 30-50). The person I purchased the snake from kept it in his unheated garage, with food and water.

If I do this, do I just keep fresh water in the tank? Do i need to offer food monthly or every third week?

I am not planning on breeding the snake.

Thanks for your advice.

If you are not planning to breed the snake your don't have to brumate them. I'm not a pro with milk snakes, but I think 30 degrees is way too cold to brumate, and would probably kill it. You want to keep a brumating snake in the low 50's. If you decide to brumate, stop feeding for a month, then lower the light and temperature every day for 2 weeks, at which point you keep it in the dark at 50-55 degrees for 4 months. After 4 months start upping the light and heat until you are back to normal. During these four months offer only fresh water weekly or when soiled. (Now someone correct this post.)

It's correct if you do not intend to breed him then brumating is not required. However, considering he is mature and has been brumated in the past, you'll need to monitor him as the days get shorter, he may start refusing food and seeking out the cooler end of the cage in an attempt to prepare for a brumation his biological clock is telling him it's time for. Should this happen it's better to go ahead and cool him just to keep him from losing weight.
Four months isn't necessary though. I normally brumate breeders for 10-12 weeks, but for a non breeder trying to brumate, 6 weeks at 50-55 as mentioned is probably enough to get him back on track.