It depends on your measure of adulthood, I guess. Guineas are sexually mature at a few weeks old, but giving baths, trimming nails, etc. tends to be recommended after 3-6 months, so in many ways that's when they're considered "adults" I guess. However, depending on breed and individual pig, I believe they can keep growing up to 12-18 months old.
I top up morning and evening and don't really measure -- one of my pigs is only 6 months old, and the other is a runt so I'm not worried about weights. It's also winter and although my boys live indoors my house gets cold, so I let them pile on as much as they can!
The bag of food will tell you how much you should be feeding -- I think it's normally about 40g per pig per day. I would say that's about what my two eat up even with unlimited access, though, and Machu especially is a greedy guts about his pellets. Unless they're ignoring their hay (which should be 80% of their diet) in favour of the pellets, I wouldn't worry too much about their intake until they're about a year old, personally. At that point you need to ensure they aren't piling on weight.