4-6 years sounds about right in line.
Even in a straight tear down, there's always the salary cap and therefore always a few guys who are untradeable on their current contracts, gotta watch those out.
Draft lottery luck is a huge factor, it's the difference between getting Kane and Van Riemsdyk, or Matthews and Laine, or Hughes and Kakko.
Free agency is also big. Not to dunk on Buffalo here but I feel their lack of appeal as a market, merited or not, adds years to their rebuild.
Ideally I imagine it should go like this.
1: Sell at the deadline and aggregate picks. Make minimal moves in UFA.
2-3: Bottom out, top 5 pick territory, 10+ picks a year.
4-5: Incremental in house improvement, be more ambitious in UFA.
6: Bubble or WC team.
As much as I hate to I do have to give the Habs credit. They shot for the moon in the draft instead of going safe, they made risky, audacious moves that succeeded (Monahan, Suzuki's deal, Laine to a degree) and backfired (Dach, Newhook), and even though Washington will probably beat them, they're back in the dance with a great crop of youth and even more picks coming. Not saying they'll be Cup favorites in 3-4 years, but they'll be a good team.