How often to backup?

I have been spending a lot of time thinking about our backup solution lately. I am impressed by some of the new solutions like the bdr backup device but not impressed by the price. It backs up every 15mins which is that really necessary? Maybe it is maybe not. I think the price is outrageous considering its just a simple server with software. Never affraid to try it on my own I have been looking to create my own server that does the same thing.
1. Backs up multiple servers to a single server.
2. Will virtualize that server if it fails.
3. Will send backups off site.

Currently we backup nightly and it has been tested and works as planned. It will even do a bare metal restore to dissimilar hardware. But what happened last year is our raid card failed. Cant pick one of those up locally so we had to order one. Well it took 3 days to get here. So I had to setup a temp server in the mean time and that wasn't fast either so I am looking to a solution to avoid that.
I know tamonline is an option but it isn't a great option for us as we use our server for more then just tam. So do I just stock a whole spare server just for parts? Which seems expensive and wasteful for it just to sit there.

More to come as I try to solve these problems.

Let me know how you have solved some of these problems?

Josh