Just one word to assure the current "assumptions":
Just logging in should keep your account alive. 🙂
It not only should, it will 🙂
The period is in fact 180 days for active accounts - that is: for accounts with a download/upload of more than 2 GB. Not active accounts will be pruned 90 days after their last login to the site.
But you will also get a reminder mail around once a month after your last login to remind you to login to our site.