Can you believe that I don't know the answer to this? This is because I have never TIMED any progress. Put another way, "a watched kettle never boils" so I don't watch it. So for every journey I've been on, be it weight loss or hair growth, I just focus on the now and do what I need to do w/o worrying about how well it's working for me or how long it will take. That's also why I never set deadlines. I let the results surprise me.
So when I got to SL, that was a surprise. I had no idea I was already there. Actually I discovered I was at SL when I was already at CBL (which means I had been at SL for quite a while since we all know SL starts from the end of NL and then because of the slant of shoulders, you stay at SL till you're at CBL which is aka Full SL).
Then APL was also a surprise. I was redoing a twist and I thought my hair felt like it was stretching longer than I remember...so I went to the mirror to look and voilà! I had made APL!!! So far, this was just the nape hair I was stretching to the length marks. When the area above my ear reached APL, that was also a surprise--again, discovered while redoing twists.
And the only reason I knew I was at BSB was when someone asked me how far my hair stretches and I checked. So for me, none of the milestones have been "a long way coming" because I didn't squint to try to see them from afar. I didn't think about them at all. I just tried to enjoy the journey and each achievement found me by surprise. So for me, even if I don't have a fast growth rate, I have not found any of the length milestones challenging at all.
So when I do get to MBL or WL, I will not be able to say how long it took because I honestly don't keep track like that. It will all feel like it's been a seamless trip coz I
ain't sweating it.