How Many Program Revions to Keep

I would always try and keep the shop laptop bang up to date. Daily if possible weekly at least. The revisions we try and keep on a separate folder ours is on the maintenance/technicians server. Annually, usually in the holiday shutdown we do a clean up of the server but keep just the latest version.

I would keep them all, but separate the current and perhaps previous version from the old versions.

(External) HD's are so very cheap these days, why not keep the old ones around?

Use version control software, it's free/cheap and you can get free/cheap storage on the internet (in case the building burns down or something). I like Git and use BitBucket for cloud storage and SourceTree to make Git easier to use.

To start a repository after you already have a hundred old versions, you copy the oldest version into a new repository with just the project name, no revision or date in the name. Commit it, then replace it with the next version but with the same file name, commit, repeat, until you have the complete version history in the repository. The repository will take up MUCH LESS SPACE and if you use decent commit messages, it will tell you what changed from version to version.

EDIT:
It will also have every revision from the beginning of time, just in case.

In addition to keeping backups on a PC, it's VERY important to keep a mirrored backup on a server or something more secure. (HDD's like to die and take backups with them).