How many years did Natsu train with Igneel? He was just a child when he joined Fairy Tail. When did Igneel decide to train Natsu and for how long?
We are shown both his appearance with Erza, Grey and Lisanna as children so it can't be very long that he trains under igneel. His appearance with igneel and with the group are very close, if I remember, practically the same.  At a maximum, I would suggest 3 years but I don't think it would be less than a single year. It's theorized that Natsu was around 8-9 and then Natsu was 11-12 when he joined Fairy Tail.  We don't know for sure though 


  Sources: http://fairytail.wikia.com/wiki/Igneel  http://fairytail.wikia.com/wiki/The_400_Year_Plan http://fairytail.wikia.com/wiki/Natsu_Dragneel http://http://fairytail.wikia.com/wiki/Fairy_Tail_(Guild)