Forever! Lol. I think it's normal for them to get distracted and take a long time to eat (even I am still a slow eater and when I was younger was often the last one left at the table and everyone else had long gone! Lol). They are easily distracted, so if he is still happy to have food put in his mouth then he probably still wants it, when he starts turning away or getting grumpy that's a pretty good indication they have had enough, or just throwing it all on the floor like DD (especially lovely when it is lasagne )

try not to let meal times be hard work, at this age food is supposed to be fun, so don't worry too much about how much he is/isn't eating and how long it takes