I thought these figures are interesting (2-3 years for college draftees, and 3-4 1/2 yrs for HS draftees) and may provide some guidance for the amount of "patience" needed.
(This is from this week's Ask BA - Jim Callis wrote the response)
When I was working on a College Vs. Pro feature back in 1990, I did some research on 1981-84 draftees. On average, the high school players who reached the majors required roughly 1½ years longer to get there than the collegians who made it to the top. Of course, the high schoolers generally began their pro careers three years earlier.
I asked someone who did a similar and more recent study for a big league team, and he got the same results. He focused on players who had significant careers, and he found that the collegians in that group spent 2-3 years in the minors, compared to 3-4½ for the high schoolers.