What was old is new again

I was cleaning out some junk in the attic and came upon a sheet of paper with my old email account from Yahoo. It was way back to year 2002. Just for fun, I tried to log back in using one of my old passwords (it was my girlfriend's name at the time). It worked! This account has been lain dormant in the Yahoo server all these years. Well, the account was still active, but all of the emails were purged. I emailed my ex girlfriend using this account just for fun. So far, no reply yet.

I wonder how long Yahoo keeps their inactive email accounts.