We vaguely started at 5-6 months with *milk* but around seven really worked on being consistent with about five or six signs. F signed a couple back by nine months and we started introducing as many signs as we consistently could at that stage. By one year she had 100 signs, now it's closer to 150. It really went in explosions - some weeks she wouldn't sign much new, other days she'd do four or five new ones.
Anyone else find that their LO likes to learn things in topics? eg F associates sun, moon, stars, sky and clouds, so very quickly learnt all of those signs together. Another thing she LOVES recently is to learn to sign a song or nursery rhyme. Eg for Hey Diddle Diddle, as I say it she'll sign cat, fiddle, cow, moon, dog, laugh and spoon. So cute!