Or longer, really; while everyone, of course, loves the 2015 MBP, they're mostly thinking of the integrated GPU one; the discrete GPU version was pretty thermally challenged. Arguably Apple's real problem with the post-2015 ones was that Intel stopped chips with fast integrated GPUs, so Apple put discrete GPUs in all SKUs.
While I don't think it's a likely theory, 5 years from "we should do it", to design, to testing, to preparing fabrication at scale, to design of the system around it, etc doesn't sound unreasonable. I would be really surprised if Apple decided the Arm migration after 2015.
Or longer, really; while everyone, of course, loves the 2015 MBP, they're mostly thinking of the integrated GPU one; the discrete GPU version was pretty thermally challenged. Arguably Apple's real problem with the post-2015 ones was that Intel stopped chips with fast integrated GPUs, so Apple put discrete GPUs in all SKUs.