Using a long-tail cast on is a really quick and easy way to cast on a large number of stitches quickly. The only problem is, you have to estimate how long of a tail you need before you can start.
Is there anything more frustrating than casting on a whole bunch of stitches only to find out that your long tail isn't long enough for all the stitches you need to cast on?
There are a couple of different methods to help you estimate how long of a tail you need. The first one is great if you have a tape measure handy; just measure out one to two inches for every stitch you need, depending on how big the needles are. Bigger needles have bigger stitches and thus require more yarn.
The other is not as scientific but works if you don't have any measuring devices handy. Just wrap the yarn around the needle you're planning to use, one wrap for each stitch. Add a few extra inches at the end for good measure. You could also wrap the yarn around both needles for a little extra insurance.