It depends upon the current. How many amps are you going to try to push down the wire? The drop is over the length of the CAT 5, however long that is. You now know that it has a loop resistance of about 0.164 ohms per running meter. Multiply that times the number of meters to get the loss in one pair. Divide by the number of pairs you are going to use for one voltage to get the total resistance.
Now, you can do several things. For instance, if you can stand to lose half a volt over the run, divide that by the resistance to get the maximum current you can pass down the cable. Or if you have to pass a specific maximum current, multiply the calculated total resistance by that current in amps to get the amount of voltage you will lose in the wire.