I night-weaned dd when she was 15 months old - something I never imagined I would do. At the time I was pregnant and she had gone from sleeping 3-4 hour stretches in the night to breastfeeding every hour, or even staying attached all.night.long.
We were both getting pretty strung out and I was hating breastfeeding her!
Dd is very strong-willed, and often would get quite worked up at bedtimes anyway - sort of seemed like she needed to have a bit of a meltdown to get herself transitioned to sleep. I also suspected that since my supply was low, she was not truly getting satisfied with a good tummy-filling feed. She would nurse, thrash, bite, nurse some more, go on and off half a dozen times. . . repeat.
So - I breastfed her in bed, and then left her with Dp (she was used to being put to sleep by him when I wasn't home - I was on-call for births as a midwifery student) and she cried and thrashed a bit, but eventually went to sleep. I never felt she was super distressed - but then again, I was pretty desperate, so maybe not the best gauge. . . .
After about a week? maybe even less? she would go to sleep quite quickly, and - surprise! stay asleep all night. At times she would spend the night sleeping draped over me, but she wouldn't wake until early morning. I hadn't really intended to 'night-wean' - jsut cut down on the falling-asleep-nursing-marathon, but it seemed to re-set her body so that she wasn't constantly fixated on the breast all night. After that, she and I were a lot more rested in the morning.
When ds came along 5 months later, she went back to occasionally night nursing. Most of the time it was fine, and sometimes I asked Dp to take her instead. Then as she got older, say 2.5 we started talking about the milk needing a rest at night, and that she could have milk again when the sun came up, and to my surprise, she was very amenable to this idea. Even if she woke during the night, crying for milk - I would gently remind her the sun would be up soon and offer her a cuddle and she'd accept and go quickly back to sleep.
Ds is 16 months now and I've made no moves to night-wean, but he's a very different nurser - much more laid back and only wakes 1-2 times a night to feed as a rule.
So, no hard/fast rules I guess. Just working with what you've got in the moment. . .