Keto is sustainable long term. I know a few guys who have been doing it for years. If you're doing it for weight loss, you just are a little more strict on your carb intake, otherwise if you just want to maintain, you can relax and probably do like 30-40g of carbs to keep in ketosis.
Anyone new to Keto I highly recommend reading the following:
http://www.dietdoctor.com/lchf
http://www.reddit.com/r/keto
http://cavemanketo.com/keto-meal-plan/