It depends on the situations you are in. I would buy a 200-300 feet of cable and the appropriate connectors and make my own. Make 2 50' cables, 4 25' cables, 2-4 10's, a couple real short ones for you mixer bag, a couple for the boom pole to mixer, etc. That way you have a bunch of cables that can cover most situations.
On the other hand, if you know you are going to have 8 mics up, you better beef up on the cables.
When I record recitals and stuff with 2 mics, I typically bring 6 25' cables. Sometimes I only use 2, sometimes I find that it helps to be +50' away, so those extra cables help.
I'd say the most versatile cable length would be 25 ft. I'd have at least 4 or 5 of those. A couple of 50's, a 100 footer, some 10's , 8 3 footers, and maybe 3 or 4 18 inchers. Get handy with a soldering iron, because you may want to put some right angle connectors on your cables at some point, and why pay someone to do it when you can do it yourself.
For mixer to camera, you may want to go with a 25ft breakaway cable. Some people get the 50, but I think that's a little long, and you can always get an extenstion after the fact.
Don't forget about adapters and turnarounds. You'll want to have an arsenal of those at your disposal. Coming up with that weirdo turnaround at a moments notice will make you look like the hero.
oh yeah, theres a universal truth if ever there was one. i was at a shoot the other day where suddenly its sprung on sound department that we would need to interface a camera to a billboard for playback. of course, production had forgotten to organise adaptors (for sound OR video).
damn it felt good when i had the right connectors to make it work.
cables wise, i dont own anything longer than 25 feet, but all told I probably have about 10 or so of those, and several is shorter lengths. I probably have about 20 or so cables lying around, although I typically one short cable for boom/mixer on eng, with a backup and 2 25footers for weird setups.