You need to thoroughly spray till the sides fog up then cover the top for half an hour. I also like to give the outside area a spray as well just to be safe. If they keep comming back its because you are not being thorough enough. If you have more than one reptile, treat them all and make sure to do it once a week for 2-3 weeks as instructed because of the life cycle of mites. LLL reptile has a good video and directional use on their site, it should pop up if you just type prevent o mite in google. Edit- also are you positive its takes several months? It's more likely thays when there's enough of them to spot, they are very small when they first hatch and when they feed they molt and get bigger. It can take 19 days for them to starve and find the way to the snake before then if im not mistaken