No there is a set recommended max distance in the regs.
I got shot the question in a job interview believe it or not and its really got me going to find the distance - it's defo in mm and its something like 50mm - 100mm etc.
"Vent pipe shall terminate not less than twice the internal Diameter of the vent pipe above the top of the float operated valve or top of the overflow pipewhichever is the higher"
Edit: the pic shows the top of the float valve being the very top of the white plastic water spout. The top of the Overflow being the top of the overflow pipe (obviously)
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