It's been a while since I've done it, but my guess would be 180-190 miles.
It's around 40 miles shorter to run thru Baraga/Lanse if they have snow. The downside is you wind up running the Portage from Chassell up or ride back roads from Chassell to the Bill Nichols.
If you have extra time to play with & like the twisty bits, running 109 to 3 instead of 12 is a fun alternative.
If you get sick of riding the grade by the time you hit Twin Lakes, the Freda Loop is a great piece of trail.