Ahhh gotcha, what is your pipe made of and what size is it. It's gonna come back to how well it transfers heat. Even the type of soil you have it buried in. But, short answer , probably about 275 feet of 6" pipe I would guess. And that is a guess. But , if you overkill it wouldn't hurt anything. Lay two pipes in your trench with some blocks to separate them. An outflow and an inflow to return. But keep in mind that is a guess , I feel good about it. But, I realize I have no skin in the game. Maybe we can get someone to chime in. PM Dr. Henley if he don't know he will by tomorrow.