We started around 9 months and only got out first sign back at around 14 months...but what I found is that he was understanding the signs I was doing to him well before he started to sign back. Then one day I asked if he wanted a drink and he nodded yes and I said 'Well, can you do the sign for me?' and he did it! I also occasionally help his hands into the different shapes, that seems to get the sign to stick quicker. Consistency (seems like a buzz word round these parts!) is key, it takes a while for you to become proficient at remembering to do the signs, and you'll have some points where you will think its never going to happen but it will, there is no doubt about that.
And when it does its amazing, we rarely have any communication problems like some of my friends with their kids. And I truly believe that DS experiences far less frustration then other kids cause he can tell me what he wants. I also find that i talk normally to DS, I tell him what's going on in quite a lot of detail and now we are at the point where I can hand him a tissue and ask him to wipe his nose and he does it all on his own (he's 16 months now) or ask him to put something on the table and he will. I am not sure if the signing has anything to do with that but it must help with overall comprehension...