I think Fed's limit will be 2016 Olympics. He is on the record as saying he wanted to play at a high level long enough so his twins could watch and remember watching him play at that level. So a few more years yet.
Yep I think that's a good assessment. He has said that he wants more kids. In the twins' first year he said in a few interviews (quite proudly) that he was hardly away from them for more than a day (even though they're too young to remember that). He noted that when they start school the girls will have to stop travelling with him everywhere. And it seems he doesn't want to be an absentee dad. So put all that together and I think another 4 years is the max. He could go into a sort of 'part-time' mode after that; playing only slams and a few favourite tournaments. But what will that do for his form, if he only plays 7 or 8 tournaments per year?