Android backups for devices that have not been used in two months can be automatically deleted by Google with no notification, as Redditor Tanglebrook recently found out the hard way. Tanglebrook had to return their Nexus 6P, and found after a few months of using an iPhone as a temporary replacement that their Android backup had been automatically deleted. There was no notification that it was going to happen, nor was there any sort of option given to use free Google Drive storage to store the backup in their own space. There was no way to recover the backup, according to a Google representative that Tanglebrook spoke to.
Google's official policy on the matter says that backups of devices that go unused for two weeks can be marked for expiration, but did not say exactly how long that expiration period was. Using the device that the backup originated from or migrating the backup to a new Android device would mark it as active and reset the timer. In Tanglebrook's case, the countdown for expiration due to inactivity was two months, but since Google does not lay out any specific time frame in its official policy on the matter, this figure could vary between users or devices.
There are a number of backup options out there besides Google's default option, which a user has little control over. Users can use the adb-based backup option, but this is useless for transferring between devices because of encryption. Similarly, backups done with Samsung and LG's solutions cannot be restored to a device from a different manufacturer. The most powerful and controllable backup solutions, however, require root, and in the case of TWRP, even require unlocking your bootloader. Titanium Backup is easily the most popular solution to backup app data, SMS messages, and other things on a rooted device. If you're willing to go as far as unlocking your bootloader and flashing a custom recovery, the TWRP recovery has a built-in backup option that can back up app data, system settings, and other things as a user chooses, or even back up the full system image in case of a catastrophe. Encryption is perfectly optional with this solution, and it can even be used to restore some types of data to other devices by storing the backup on a MicroSD card, though doing that can cause issues. On the whole, Google's default backup will likely best serve the needs of the average Android device user, but users should keep in mind that it has its caveats, just like any other backup solution.