It was about 3 months.
They recommended repeating whatever he says, so I'm not correcting his mistakes, but he is hearing how the words should sound. So if he says "See Soo Fom" for example, you could say "Yes, Ds, let's watch Tree Fu Tom!"
They also said to narrate everything you do together. So if you're playing with cars and a garage, you talk about everything that's happening. "The blue car is going up in the lift, let's drive it down the ramp really fast. Wee! Now it's the red car's turn."
I just chat to him constantly, all the way to the shops I'm pointing out stuff and asking what he can see. I just have to be careful not to bombard him with too many questions, and try not to answer for him when he struggles, or correct his mistakes.
As he's getting older (he's 4 in May too), he's starting to understand that if I can't understand what he's said, him getting frustrated and shouting and crying at me doesn't help. He sometimes has to go and sit and calm down, then try to tell me again in a minute. We usually get there in the end, but it's so upsetting for him not being able to communicate sometimes. Since he started at the infant school nursery his speech has really come along, but it's hard when he's telling me about something that happened at school and I can't work out what he means as I have no context to work with!