I think it really depends on the health of your hair and ability to retain you ends. Remeber all of the damage you imposed on your hair with constant blowdrying, excessive heat BEFORE joining LHCF? Well the results of that damage are sitting on your ends and they are ready to break off the fastest and first.
In my personal experience, I had to go through the process of trimming the breakage from my ends and previous damage before I began to retain the lenght. So it took about a year and a half before I got to a healty armpit - and I feel I am still struggling.
SL to armpit for me was very challenging not only because of trying to salvage the ends from my hair frying days but also because if I did not wear a protectve style it during this period it constantly brushed against my shoulders causing breakage. Watch out for this ladies.
If you are at a point at SL and you have healthy ends then I would say between six months to one year. Otherwise add another six months.
Not to be discouraging but more realistic. I have learned this NOW after 3 years of hair care that I need to set realistic goals. Give yourselves an additional 6 months to your target date (I know it can seem like eternity) so you wont be dissapointed. You will be surprised at how many setbacks (learning experiences) you go thru on your journey. And hey, if you meet your goal prior to that its all good...an early surprise!
I thought by now I would be a BSL - shooting for July. But after examining the condition of ends even after being good 90% of the time, I feel the need for a nice inch trim. This would push me back to September for a solid BSL.
Good Luck!