By my math, you have full turnover in about 170 weeks, or 3 years.
The key thing in figuring you recovery time objective (RTO) is what data your users need to be productive. Since this is video files, I'm guessing that restoring the most recent data first will get them productive while you bring the rest of the library online. But only you know how often the older stuff is accessed.
You'd be surprised how well tape can handle a job like this. Video files are big and long, and if they're not fragmented all that much can stream very fast. And importantly, will restore very fast since it's lots of big, sequential writes. A weekly net-change tape cross referenced with a database to track what's on each tape could give you a sizeable offline archive if you want.
If your videos are of the write-once-read-many variety, you can go a long, long way by just doing a weekly changed-data backup coupled with a data-replication solution. It would give you the 'instant recovery' of a full replicated solution, but with an alternate recovery method in the form of the tapes. It'll mean doubling your direct storage costs, but you can't beat the time-to-recovery of having a hot spare.
If a hot spare is too rich for your blood, the cost-per-GB of tape is still well below that of disk. It'll take longer to recover, and take a long time to fully back up, but it'll get you there in the end.