90k miles is a pretty decent amount. It'll equate to about 4 upgrade tickets. It take 25k to upgrade between classes.

The miles will expire after 1 year (18 months?) of inactivity. So if you don't fly, you can always buy 1000 miles to keep the acount alive. Alternatively, if you have the BA visa card, any purchase will add miles to the account and keep it alive.

R