These "transitional 55mph zones", at least as I've observed them thus far, are quite long.
On the north end, it starts at Ebenezer Dr south of Watertown, and continues as 55mph south to River Dr. in Johnson Creek. By Google Maps' reckoning, it's about 3 1/2 miles. If they extended the 65mph section farther south since my last visit, the logical transition point would be at either Spruce Dr or Baneck Ln.
On the south end, I'll charitably calculate it from County Y south to Jefferson Rd/Earl Ln, about 1 1/2 miles. The 45mph section might extend south of the new railroad overpass now; it was hard to tell the last time I went through there.
Much as I'm confounded with the strip of WIS 26 north of I-39/90 in Janesville, I'm confounded by the Johnson Creek section. WISDOT spent a boatload to upgrade and reroute the highway everywhere else, but then left this choke point without an obvious plan for remediation. The worst part is that WIS 26 through Johnson Creek can be upgraded to freeway right up to the I-94 interchange, but seems destined to be a traffic-choked commercial strip for at least the next 20 years.