UPDATE: Microsoft has provided VideoGamer.com with the following statement in regards to this story, clarifying that players will continue to have access to their purchased content provided they still have an active Microsoft account.
"The new policy in the Microsoft Services Agreement clarifies that after five years of inactivity on Xbox Services, the Gamertag may be repurposed," a Microsoft spokesperson said. "In that instance, the user with an active Microsoft account, and an inactive Xbox account, would still have access to Gamerscore, licenses and friends as long as the user is still able to access their Microsoft account."
As originally reported, you must log in to your Microsoft account at least once every five years to keep it active.
ORIGINAL STORY: Xbox players now need to sign into their Xbox Live account at least once every 5 years to keep hold of their gamertag.
The changes were revealed in a revised version of Microsoft's Service Agreement and come into effect from September 15. It isn't yet clear, however, whether players will also lose all of their purchases attached to their Xbox Live account if they don't sign in within the time limit, although it sounds as if they will.
A line earlier in the agreement states that users "must use your Microsoft account to keep it active. This means you must sign in at least once in a five-year period to keep your Microsoft account, and associated Services, active, unless provided otherwise in an offer for a paid portion of the Services.
"If you don't sign in during this time, we will assume your Microsoft account is inactive and will close it for you."
Furthermore, if your account is found to have been compromised, Microsoft says that it "may be required to disable access to certain content". It isn't clear which content this is or whether it will be permanently or temporarily disabled.
You can read a summary of the changes made to the Services Agreement through here, or take a look here for the full thing.