The number of years it takes depends on a number of varying factors.
So from what the title of your post says, I take it that you're 4.1" and you want to get to 5.5" - is that correct? So you want to gain 1.4" to your erect size. That is a very manageable goal and not at all unrealistic. What will determine the time that it takes to gain that length is the routine that you perform and the length of time you continue with the routine (i.e. 7 days a week, 6 days/1 off). As I mentioned in the previous reply, please look to read and try the
SRT theory and routine by DLD (click on the
SRT text and it should take you there). Aside from that, genetics are going to play some role but not in any way to stop you from reaching your goal but just the time it takes. As a rough estimate, if you were to follow
SRT, you could reach your goal in 6 months to a year.