Average lifespan is about 5 to 10 years. The biggest indication of a weight problem would be activity level. Healthy birds are very active in the morning and evening hours, scratching around and foraging. The middle part of the day is spent mostly resting. Birds that never venture away from the food and water are very likely overweight.
Just keep the treats limited, and give them adequate run space with lots to do, and it should never be a problem. They will not overeat pellets. It's treats that really do the damage, in terms of fat build up. Lots can be done to encourage a flock to be more active. A pile of grass clippings can keep a small flock occupied for days. A stump or large rock in the run that gets flipped occasionally to reveal bugs is good, too. Hang chunks of melon, squash, or cabbage heads for them to jump and peck at. These are ways to 'treat' your flock, and at the same time, make them work.