What Happens to Dormant Gmail Addresses?
What happens if you have a second (or third) Gmail address (i.e. Google Account) that you don’t often use? Well, after 90 days without use it’ll be deleted. Here’s what Google has to say:
A dormant address is a Gmail address that hasn’t been used for six months.
You can still receive mail if your address is dormant, but you need to log in to keep your account active.
If you don’t log in to Gmail within three months of it being labeled dormant — or for nine consecutive months — Google may delete the address.
via Dormant addresses – Gmail Help.