Hi Frank!
I never delete projects as they are only reference files and doesn't take up that much space. I just organize them in folders for years/months etc. I run a backup now and then and include stills.
Backup of footage is regulated in the contracts with our clients, but we usually keep it available on disks in 6 months only. For long term availability we have found that it's most cost effective (and safe) to save footage on LTO tapes, but that's just an extra service that our client pays for. In most cases it's more than enough to have a high resolution copy of the offline available on tape.
Thomas