I asked my HV same question - how much milk to give? She said to give it until DS wouldn't take any more! (Not helpful).
I would trial and error it, but maybe 6-7oz (mind you, will depend on how many feeds he has over 24 hours and his size. My DS feeds for England!). I am sure every mum would have a different guess / opinion on how much!? You will work it out after a few goes and see what he takes.
I believe you can keep it defrosted for 24 hours, but I would double check this on Internet - Breastfeeding Network, NCT, Kellymom sites. I wouldn't want to get that wrong.
I have reheated EBM, purely cos it is either in fridge or as way of defrosting. But again, whatever your LO will take really. I believe either are fine, but personally, I think warmer must be nicer?
I pump for 20 minutes ish (but as I am now cutting back, I have to pump a little longer to get the same).
Also, I always freeze with bag 'lying down' (again, quicker to defrost, assuming you are using airtight bags!!?) and put all bags in a tupperware box, just in case of any leakages!
It can help to express after a bath / massage your boobs (can't think of a better word!) before expressing and some say to think of your baby/having picture nearby (personally this didn't make any difference to me!).
You will find that you will make more, the more you express.
Good luck !