Free accounts left abandoned for six consecutive months, are subject to being deleted. As long as you login to your account at least once every six months (perhaps five months would be a safer bet), this will not effect you.
We are often lenient about the six month rule, so please don't rely heavily on the fact that inactive free accounts are deleted on the six month mark. They may be kept alive for much longer. It's not an automated process, we simply go through from time to time and clean out abandoned accounts.
Does This Apply to Paid Accounts Too?
No. When you pay for an account, you pay for a full year. It does not matter if you use it or not, it will not be deleted during that year.
What If I Stop Paying for My Account?
Should your paid account come up for renewal, and you chose not to renew it, it will be considered a Free account again. So if a non-renewal is followed by six months of inactivity, it will be subject to deletion just like any other Free account.