He'll start by learning the essentials. In my DS' case these were: mine, don't, meat pie (), give it to me, I don't want to, I want to. This took him a matter of days.
Then over the next few weeks I think it was a case of managing to understand what was being said to him. The nursery teachers were great in making sure he grasped most of what was happening. They had some experience of foreign kids already.
Then I would say it took up to six months before he was noticable just getting on with it and playing and chatting freely. And, very importantly, being understood.
This past school year (he's been 4 yrs old) has been a big leap forward for him in terms of expanding his vocab and communicating (i.e. imitating the other kids' words, accents, expressions), helped along quite a bit lately with playing with kids outside of school.
I wouldn't say he is fully fluent but to listen to him now, talking in the vocab of a 4/5 yr old, you would find it difficult to distinguish him from other kids.
Really, the best thing to do is to just go for it and not worry about it. Kids that young will adapt and will just know how to separate the languages according to who they are talking with.
My sister and I (when aged 10 and 8 respectively) learnt a new language from scratch within a few months because we were chucked in at the deep end - it was a summer club in the country we moved to and we had no choice but to learn before the school year started - hardly anyone spoke english. By the time a year had passed we were fluent. We still speak the language fluently and believe it was the best way forward. Other foreign kids in the same town who went to the international school (studying in english) were barely able to communicate in the local language.