I weigh every week, or once every two weeks. Daily doesn't work for me, it holds too much power of re-triggering my old bouts of ED, so I don't want to touch that with a 10-ft pole now. But I mostly know now if what I eat is going to wreck havoc or not, so I keep aware about that, and balance things out over the course of a week.
Anyway. More than 1 kg (~2 lbs) gained means I have to be careful for the next few days. I like joking about nipping it in the bud because being careful for a few days is easy, but on the other hand I'm way too lazy to go on a diet for weeks.

(I'm also too lazy to walk to the nearest McDonald's; my kind of laziness is actually great.)
These days I'm not sure where to put my lower limit, though. I thought I was alright in the 58-59 kgs range, but after my teeth problems, and having to reconsider a lot of food-related things (due to being unable to eat properly, that is), I've noticed I can easily maintain in a 55-56 kgs range, without having to restrain/deprive myself. I'll revise my 2-lbs-up limit accordingly if I see this is something I can indeed keep up without having to resort to unhealthy means.