nogales to sayulita is 3 days
w/ over nights in San Carlos and Mazatlan.
that part of Mex is boring however and hiway toll roads.
have you considered entering in Douglas and taking 17>14 or Rio Sonora route to Hermosillo? you could also bag san carlos and ride out and back east on 16. no time frame right?
on the way back west on 16 take 12 south and then head to Mazatlan.
other options are take 16 east and then hit copper canyon and the mtn road south to durango and then devil spine to maz or the alternative paved route that SR (live in mexico) know about and come out farther south.
either way you drop down to the coast at san blas exit ans then it's costal roads to sayulita. make sure to stop in chacala along the way and maybe spend a night!