snakes can go quite some time without eating. during hibernation, though they have low metabolisms, several months without eating is normal. Since it's not hibernating I wouldn't be too concerned until 5 weeks. They don't typically eat when they are about to shed and they don't eat when they are gravid usually. A better way to look at it is how much of their body mass have they lost? Some people start force feeding after about 30% of their mass has been lost. Since you probably don't have a starting mass to go by then I wouldn't be too concerned until it starts to look pretty scrawny. That could mean one month, it may mean two. Again, the point where I get worried is 5 weeks. I'm no expert though and it really does depend on the snake...