1/8 th -3/4 in per ft
1/8" per foot
you need to know the PITCH of the drain line to give an proper answer
1/4 inch per foot.
That would depend on PITCH more pitch more fixtures allowed
Most of the time it is 4 inch.
The building code where I live requires a drain pipe to slope approximately 1/4 inch per foot (+ or - 1/8 inch). The recommended pitch for a 50 foot drain would be 12-1/2 inches.
Wouldn't recommend it as things may back up in the drain line. 4" line is the norm ... you want to get rid of waste, not keep it around longer.
Ideally 1/4" per foot minimum
The length of a 4-inch line is 4-inches.
1/4" per foot
1/4 inch per foot.