Expire date doesn't determine how many days are kept. That's a marker for old style tape backup systems.

How big is this database? Keeping 90 days worth of individual backups could be a major undertaking if you've got even a moderately small database, say 50gb. 90 days of backing up that database would be 4500gb. Do you have 4tb of spare disk space sitting around? What if it's a medium sized database, say 500gb. Now the problem just got worse.

Most businesses would not want to go back 90 days and lose that much data. In general, you need to talk to your business to determine what they want the backups to do. Yes, you might keep a 90 day old backup, but you don't generally keep the intervening 89 days. My previous employer keeps a rolling set of backups available. One month and one week, offline, and the last two days online. In addition to that, they have the log backups for both days, also online for full blown disaster recovery to a point in time. For legal purposes there are also 6 month and 1 year backups mainained, but not the daily changes over all that time.

Since you're really just getting started in this, can I suggest you get a copy of my book, SQL Server 2008 Adminstration. It'll help you on the basics.