If you look at the first Macbook Pro model released this year, the rMBP 13, the update arrived in March. The update frequency in rMBPs average around 217 according to the Buyers Guide. If we find out the number of days between 9. March and using the old Yosemite release date, 16. October, we see a space of 221 days. Since the Macbook Pro 15 only upgraded force touch and dGPU, holding on the same CPU and clock speeds, I expect both a redesign and Skylake upgrade along with El Capitan this fall.
Three years is a lifetime in computing, so a redesign from the original 2012 retina MBP is must to keep peoples interest in Apple. The CPUs haven't really gotten a bump since Sandy Bridge, and the only way to show off some muscle is to innovate with design. Since thermal and battery is the main focus now, expect thinner Pros some time in October.
Users who need a dGPU will also need lots of space to store and render, requiring external storage, making the real utility of a laptop no lenger relevant. It's time to dump the the heated dGPU and solely depend on iGPU. One port and one mic packed in a thin and light quad core machine.