Rail length?

So ive been thinking about making a metal rail for a while now, ive found out the costs, but pretty vaguely, and I know somone with a welder so all that is pretty much covered. But i have been debating over one thing, the length. I was thinking 24', but i think its too long and im not really sure how mobile it will be because i am thinking i will be moving it to my friends house and back, and once the winter comes along, up to the mountains. So how long do you guys think I should make it?