How long should a successful rebuild take?

Its not how long will it take. The question is how long do you want to sustain it. Drafting the perfect pick at qb can do it in 1 year (see Houston). Draft the perfect pick in the correct year (3-5) to sustain success. With the wild QB salary numbers drafting that first gives you 4-5 years max. After that you are moving parts to stay under the cap. You build the team with a decent bridge qb and draft one in year 3 you are looking at an 8-10 year window. This gives you the best possible team with a really good young qb on a rookie deal.