Ok, long story short, I'm currious how long it would take an agency to crack a 10-15 character winrar password. The file names in the archive are also scrambled including a word at the start and numbers and characters. Roughly, should I have reason to believe this could be cracked within a reasonable time frame?
Then it depends, not on the length of your password, but on the way you produced it. It's not the length which makes the password strong, but the randomness. If your password is a sequence of 15 random characters, chosen uniformly and independently of each other, then it will resist forever. If it is a common English word which happens to be 15 characters long, then it is toast and will be recovered in a matter of seconds.
As mentioned rar uses SHA-1 which is pretty strong. You can have a good comparison of different algorithms here. As you can see cracking rar is slower than WPA-PSK.