it is common and normal for babies to fall asleep on the breast because its just so comfortable being so close to Mommy. Both my boys did the same too.
For me, whenever they fall asleep, I'll just put them down. Yes, it will be very tiring for you, 'cos with only 10-15mins of suckling time, the baby might not be full yet. Then again, it depends on individual babies. Some, like my younger one have big appetite, I end up being a cow for almost 24hours non-stop. *faint*
Because breast milk is easily digestible, by expressing it out, I can't really give you a exact quantity. All I can say is, you need to get to know your baby by emptying your breast for the first time, then feed him, take note of how much he consume, over a period of 5 feeds. If he usually stops after 100ml, then you know you should pump out around that amount in future.
Personally, I do not like to express. 'Cos there is bound to have wastage and breastmilk is so precious and needs time to reproduce. So hhhmmm.... expressing is not a choice for me. *wink*
Good luck girl and congrats!