I have always had days were I looked pregnant when I wasn't at all. I put this down to bloating and certain times of the month. So I felt I was showing from about 10 weeks but one day I looked pregnant and the next I didn't so that is why i think that. But I am 16 weeks today (yeah) and just the last 2 weeks specifically the bump has stayed very consistent although there are days I feel huge. So I think there is still an amount of bloating.
So I would say though that I have started showing around 14 weeks.