You are allowed 1/3 the vertical for the horizontal runs. More than that and they like to see it bumped one size.
I have a question about venting a kitchen sink in a new location. My kitchen remodel plan includes moving the sink away from its current vent to a different location on the adjacent wall in the kitchen. It would be possible to run a new vent up to the roof through the attic space on the 2d floor above but would require a horizontal run of 6 feet to do that, although I would hate to penetrate my clay tile roof. Alternatively, it appears that my other option would be to run an accessory vent starting from above the sink then run it horizontally 8' through wall studs then make a 90 degree turn to run through another 8' of studs (no studs that I mention are load-bearing) to tie into the current vent. It will be possible to properly pitch any horizontally lines as necessary in either scenario. Any thoughts? Happy to clarify if my description is unclear.