As everyone has already said, it varies from person to person and even pregnancy to pregnancy. I ate like a horse and did zero exercize with my first (due to a debilitating back injury) and only gained about 8kgs in my whole preg. I was bigger to begin with and my understanding is that if you're already overweight, you gain less because your body 'knows' you don't need the extra fat stores for breastfeeding. The weight didn't come off easily for me at all despite breastfeeding, i weigh 5kgs more now than I did at the start of my last preg! So even if you do gain more than you were expecting that doesn't mean you're unhealthy or that you won't be able to lose the weight.