The gas line is leaking on my car. I called a shop and they are asking 70 bucks a hour plus parts to fix, but cant give me an estimate because they say they dont know how long for sure it will take. I would like to know how long a job like this would take and what the cost of the parts could be.

They're telling the truth. A '91 is going to be very rusty underneath. The fuel line also runs right next to the brake lines, so there is a good possibility that one of the brake lines will break while attempting to loosen a fitting on the fuel line. Worst case scenario, probably about 4-5 hours to drop the tank, break the straps, and replace the lines all the way up (assuming nothing else breaks in the process). I doubt factory parts are available, so it will probably be pieced together. Either pay them the $70 to find the leak, or do it yourself. At the very least you can see what is leaking, and evaluate whether or not you can do it yourself.