How long?

Does anybody know the longest effective mass for a 8" RMH.
Im wanting to build An 8" RMH  with a 55 gallon drum and heat mass. into the new green house I'm putting up for the wife this year. And depending in the size of the mass will determine it's positioning within the green house, ideally I would run the mass for as much of the length of the greenhouse down the center, with a potting seeding bench above the mass, and grow platforms around the outside.

Any help and advice is greatly participated.

Thanks Phil

50' using 8" pipe , with a 5' deduction for every 90 degree bend.  Or using the matt walker 1/2 barrel system you can have 5 barrels end to end.