As astronaut is always to choose the food of his/her own choice . They have a tasting period of 15 days to eat and choose what foods he/she likes to eat in space. ofcourse there will be moderation by nutrition experts to ensure the astronaut is getting the recommended diet value of car/calories and so on.
Kalpana Chawla - A vegetarian stayed in space for close to 31 days. There are few other people who stayed much longer than this.
P.s : Not everyone astronaut is gonna be vegetarian but if they are going to MARS they should became vegans. The Mars living room test (underground test) for 1 year included only vegan foods.