Lucky guy. SO wants to please you. sadly rare these days.

I doubt BSL is terminal for her if it is thick all the way to the ends and assuming she is healthy and relatively young. Genetics are certainly a big factor, but at these lengths it also matters a lot how she takes care of her hair.

How often does she wash it? Hopefully not more than 2-3 times per week, less might be even better.

Very important -- does she roughly brush it when it is wet? This can seriously damage her hair. Best is a wide tooth comb used gently. Very gentle brushing starting at the bottom is OK. This is something you can offer to help with if possible. Using a good conditioner after she washes her hair helps too.

Hopefully no chemicals (color, straightener, etc.). Hopefully minimal if any swimming in chlorinated pools. Try getting her a swim cap if she swims a lot in pools.

No heat styling or minimal heat styling (special occasions only). Blowdrying should not be part of her regular routine.

Only snag-free scrunchies and hair ties. Any hair tie with a metal bracket should be tossed.

How does she wear it to sleep? If she wears it loose and tosses and turns a lot that can cause damage unless the pillowcase is extremely soft (satin, silk). Most women I've known with longer-than-BSL hair have braided it at night; loose is best. This is another place you can help her -- if you don't know how to braid her hair you should learn. It's amazingly intimate and can also be great foreplay if you make a habit of it.

How does she wear it day-to-day? Extremely tight braids and ponytails can cause breakage. With thick hair it may not matter, but it seems many women with very-long fine hair generally wear it up or in a braid, both of which provide protection.

If her criteria for too-long is "can sit on it," with luck she might get beyond waist to hip length. If she gets in the habit of braiding it at night and braiding it or wearing it up during the day, it might even get a little longer before it starts to annoy her.

As for tracking her growth, it it doesn't bother her, try measuring her hair once every month or two and taking a picture of it down and brushed out from the back. You can see growth as her hair reaches various features on her body (e.g., freckles, natural waist, etc.) Bra straps move around a lot depending on size so are not a reliable metric. If making this much fuss will bug her, you can just mentally track her growth this way too without the pictures.

Good luck and have fun.