I am not certain of the absolute minimum length. To retro fit my existing firewall I cut the stand offs a bit to just clear the choke baffle. Choke baffle articulation is the limiting factor for length for all practical purposes.

I have approximately 1 mm clearance between the soft mount and the baffle. There is a portion of the carb that actually resides inside the central cutout of the mount. I also opened a hole in my firewall such that the engine actually breathes from the fuselage. It wasn't necessary since there is plenty of clearance around the mount central opening but I did it to help in the quietness department. It helps quiet the intake. It's a technique I will carry forward to other set-ups

MattK