How long does it take to get a six pack with this regime?
3 days of weight training
3 days of interval cardio
1 day of rest
and eating 5 small meals a day. Getting 145 grams of protein, 80 grams of good carbs, and about 1,600 calories. I’m 15 years old and a boy so I have a pretty fast metabolism. So how long will it take?

Not very long. A bit of advice is to eat more carbohydrates than that, if you are working out that intensely you need about 120-160g of carbs!
Everyone builds muscle at a different pace, because of different body structures, but in 2-4 weeks you should notice a difference.
Also, if you are training, make sure it is your whole body, and not one part, to create proportion. There is nothing worse than someone with undefined arms, pecs, shoulders, back but ripped abs!