Looking at
https://www.extension.purdue.edu/extmedia/id/id-170-w.html it says "A 2 percent slope is required from the house sewer pipe to the septic tank, while a
1 percent slope is sufficient for the pipe carrying septic effluent to the distribution box. To ensure that absorption field trenches are located in the more permeable soil horizons, the septic tank and distribution box should be installed at as shallow a depth as possible, with a minimum soil cover of at least 6 inches." That means it needs 1/8" per ft slope.
I don't know if the effluent pipe has the same rules about allowed length-- but the rule is usually that it can't exceed one pipe diameter in drop (otherwise it closes off the air and creates a vacuum so water won't flow).
If it does follow those rules, then the pipe length should not exceed 32'. (From what I read the required size of effluent pipe is 4" diameter).
Anyone know more on this? Am I correct? If not, please let me know.
But before you can put field lines anywhere, it has to be approved by your local health inspector and you will need a permit.
You will likely be asked to do a percolation test (unless the health inspector does it for you) to see how much length you need on your field lines. You will also need to check if there is a slope to the area-- it needs to be relatively flat.
Also, if your jurisdiction allows, you might want to consider getting infiltrator systems instead of pipe and gravel. It requires deeper and wider trenches, but does not require gravel, they can be shorter in length, and they can curve around. They are essentially 4' long dome segments that click together easily.
IIRC, you get charged per foot on the digging so the longer the run the more expensive it is. Depending on how long your field lines need to be, I'm not sure which method would be cheaper.
With my soil percolation rate and size of my septic tank, I would have needed 300' of perforated pipe (so three 100ft trenches). I ended up getting the Infiltrator Equalizer36 Quick4 instead so I only needed 2 shorter runs.
TL: DR?
Probably max length from septic tank outlet to D-box is no more than 32ft (if I'm correct).