How often to feed mice to a garter snake?

I've had the snake for 4 years, I saved her from a cat. But she would never eat mice before. She only wanted earthworms or fish ( live feeders or T/F strips of haddock).I tried T/F mice and she would always reject them. Despite this,she is very healthy -looking and fat, and is feisty as well;she bites! But I have not been able to find nightcrawlers at any of the store lately,and the snake was very hungry. So I bought 2 frozen mice today and brought them home and thawed one in warm water. She wanted nothing to do with it at first, but after about 3 hours, I dipped it in her water dish and dragged it around where I usually put her worms. She ate it! I was quite pleased as I know mice are nutritionally complete. Now, how often do I give her a mouse? I will continue to feed the worms and fish,but will rotate them with mice. Is once or twice a month enough? Thanks.