How long should I keep Lightroom Backup catalogues / databases?

Once a week, when starting Lightroom, I am prompted to backup my database. I usually agree and the backup file is saved in my Lightroom folder. I noticed that when it creates a new backup folder it leaves the old ones in place. This is probably a good thing, but I am curious about just how long I should keep the old catalogues (previously databases). I have 16 backup folders dating back to the beginning of March. The folders are getting fairly large now - between 150 & 180 MB each.

Any thoughts on how long we should keep these on our hard drive? Should we keep more than one or two backup folder(s)?

I delete all but the last one... sometimes I'll back the current one up while I'm backing up my images also.

I also write out my changes to .xmp, so most of my changes (not virutal copies, though) will be backed up regularly with my images, and all won't be entirely lost if something would happen to corrupt the catalog.

I tend to keep a couple, just in case the last one goes corrupt or something.

Thanks, that's good to know. I think I'll keep the last two backups. I already save the folder to an external drive with SyncToy about once a week with the rest of my photo directories.