Hi MishuB,
Accounts that have never been used are deleted every four weeks or so.
By "never used," we mean no forum posts or comments on news posts have been left by that account. This is the definition for "use" right now because there is currently no other use for accounts (login is not required to use any of the tools, and no information regarding your usage of the tools is recorded to your account - though this may change at some point in the future, as many people have requested progress goals and badges). This policy has helped us to eliminate tens of thousands of spammer accounts, decreasing the load on our database and the number of spam posts that get stealth posted by accounts that sit for months before becoming activated for that purpose.
If you HAVE been posting on the forums, then this is a big issue, please let me know about it! But if this is your first post, then simply by asking this question your account should stick around now. ;)