According to the FAQ;
Characters will be flagged as ‘inactive’ when the following time thresholds are met:
Level 1-5 characters will be flagged as inactive if they have not been played in the last 30 days
Level 6-20 characters will be flagged as inactive if they have not been played in the last 90 days
Level 21-49 characters will be flagged as inactive if they have not been played in the last 365 days
Level 50 characters will never be flagged as inactive
When a character is flagged as inactive they are not immediately renamed. Instead, their name is simply no longer locked. This allows another player to create a character with that name, at which point the original character will be renamed.
You can remove the inactive flag on a character at any time simply by logging them in.
Some players have like 10 or more so how do they log into all of them frequently one by one(before 30 days is up) just to make sure the character is still active and the name doesn't become available to someone else? Surely having to keep on logging in continuously would be tiring. Unless they just don't create a lot of characters and stick to just a few to concentrate on to reach lvl 50, so that the character will never be inactive?