Your data stays the same even after you log out and log back in as long as you use LiquidSky continuously. If you are on Pay-as-You-Go you need to log in at least every 7 days to prevent your SkyComputer from being deleted.
If you are on a Gamer or Unlimited Plan your SkyComputer is never deleted, unless you change your data center, reset your SkyComputer or downgrade your plan. When you downgrade your plan, your SkyComputer is being deleted immediately so please have that in mind. If you miss your payment you also have 7 days until your SkyComputer gets deleted.