I started baby sign class with ds when he was about 6 months. The teacher said at that age it would take about 6 weeks for bub to learn the sign (not necessarily sign back yet though) and by about 1 it would take 4 weeks for them to learn.
Ds learnt the signs for milk, more, food, drink & finished by about 7-8 months. At 14 months he recognises milk, more, finished, mum, dad, cat, goodmorning, goodnight, bath, duck, food, drink. He can sign mum, dad, milk, food, drink, good morning, good night...possibly more but he doesn't do it all the time (and often we aren't consistent enough).
Oh I forgot nappy, clean, dirty, clothes, kiss, cuddle and I think he has mistaken thank you for kissing as I tend to thank him for kissing me! I try to use a lot of signs whenever I can (ie. Zoo animals, book, playground, toothbrush...so many I forget them all).