Just chiming in here, IMHO I'd add that it's not a binary or linear answer.
Most of the lengthening pain can be a dull, nagging pain in your skin and nerves as the the pins tear away at them. I say it's not linear or easy to quantify/schedule because the pain can come earlier, if you catch a nerve, then go away and it may or may not come back again.
The worst pain is in the very beginning. Lengthening too much isn't any more painful, rather there's just a higher chance of complications and recovery issues
I had intense, keep-me-up-all-night pain for a bit around 4.5-5cm, then my last .75cm (past ~6.75cm), the pain got pretty horrific. I was taking Tylenol, Ultracet, Flexura-D, and Mobizox as much as they would give me. Of course, most people shouldn't lengthen more than 6cm or so.
In consolidation, all the pain is pretty dull. A lot of it for me, was in the lower back. My knee pain was mostly because of the non-union. Also, being able to tolerate the pain is one thing...being able to function with it is another. I had sporadic pains in the L knee and fibula that I could easily tolerate in themselves, but it was so difficult to walk around on it (with a single crutch) that I chose to stay home at lot...even in bed a lot of times.
As for the 1-10 scale, it's definitely useful, but still rather subjective (which is why I keep my own personal Pain Meter). I have always had a high tolerance for pain. I've participated in some extreme fighting sports (WAY beyond the pains of American football or rugby) and I will tell you: when I accidentally set my newly-broken leg on the floor, 4th day post-osteotomy, it was surely a 9.5 for a few seconds. Had I to endure minutes of that, I would have jumped out the window...you know, a 99 on the BPM.