Linden Lab Official:How long can my Second Life account remain inactive before it's deleted?

As long as you're in good standing with Linden Lab -- not delinquent with billing or otherwise in violation of the Community Standards and Terms of Service -- your Second Life account can remain inactive indefinitely. This means that your avatar and inventory will continue to be stored on our servers.

Generally, Basic accounts can be left alone if you're not going to log in for a long time, while Premium accounts need to be paid for accordingly, including tier fees for land holdings, or risk being put on hold and deleted.