The general rule is when the snake is longer than the length and width of the Viv. I moved my boys out of their large fauns into their 3ft vivs when they were a year old. How old is your snake and how long? Also what size are you currently using and what was you going to move to? As long as you provide longs of cover and are sure its escape proof then there is no reason why you couldn't move to a bigger viv