I know the carton says discard after 3 days but I have had a carton in my fridge for 3 weeks now and it still smells, tastes and looks exactly the same as when I got it.

I was using it at work for tea and coffee but throwing it out at the end of the week.

I use soya milk, that says 3 days once opened, but I keep it for a week, it still seems fine, but I just dont fancy it after a week.

If it smells okay and tastes okay it's probably okay.

Is that fresh milk or UHT? If UHT then it'll last longer as it's been heat treated to kill bacteria, the fact that it's lactose free shouldn't make any difference to how long it lasts compared to normal fresh or UHT milk.

i use soya milk too, but i never read the carton lol

It's just regular Lacto-free not the UHT one