It took me 1 year and 5 months. I had a lot of damage (from salon). My hair was different lengths (from NL to SL all over) and it was fragile. I kept my hair at SL until all the damaged hair was gone and I was full SL. After I reached full SL, I let it get longer and I just reached full APL last month.
I did this by:
Stretching my relaxers (~ 12 weeks)
PS 5 days a week (I enjoy my hair on the weekends).
Weekly deep conditions
Chelating/clarifying 1x month
Co-washing 1-2x a week (I work out so I have to)
Moisturizing daily and sealing my ends
Getting rid of split ends using search and destroy method.
I will use this same regimen to get to BSL.