Limb mending is currently random. As in, it gets checked once in a while, but when it fails, you do zero progress. Likewise, if you’re lucky, it can mend (ie. go from 0 to 1 hp) instantly when you put the splint on.
If you’re playing experimental, make sure you wear the splint on correct limb.
Factors:
[ul][li]Sleep. Helps a lot if you sleep, hurts a bit if you are very tired.[/li]
[li]Being drunk or on cigs hurts it. Being addicted to those also hurts it.[/li]
[li]Regeneration mutations have a significant effect.[/li]
[li]Being Full or Slaked helps it a lot, but being hungry or thirsty doesn’t hurt it[/li]
[li]Hidden health stat can help it a lot. I also realized that there’s a bug with it that can slow down healing a lot, will fix soon.[/li]
[li]Radiation hurts it a bit at low levels, but wrecks it totally at high levels.[/li][/ul]