How long can the exhaust port be?

I’m in the early sketching stages of a cabin with a RMH build in a couple years.  A thought that occurred to me is using the exhaust to add warmth to an adjacent green house.  

How sensitive would the combustion process be to the back pressure created by a longer exhaust?  It could probably be alleviated by stepping up the diameter of the exhaust extension, but wondering if anyone has actually tried it.  Any experience is certainly better than my “probably”.