How do you stay interested in something for a long time?

Have more than one interest/hobby! I can't imagine doing the same for months on end with no alternative. I naturally gravitate to a monthly rotation (not consciously, just kinda happens) of high interest in something, and then there's a few things I do whenever (read a book when I have a few minutes here or there, do some cross stitching while watching my husband play a game, etc).