When to pump to be able to breastfeed?

I'd like to start pumping to make milk reserves for when I'll go back to work. My little one (4 weeks old) is a voracious eater, I often breastfeed him every hour so I don't dare to pump because I am afraid I won't have any milk left for his next breastfeeding session: how long does it take for the milk to be ready for a breastfeeding session after pumping?

[KATE535]

It would be good to pump after you nurse him for about 10 mins on the side he just fed from. You may not get much initially but don't worry because your milk supply will soon adjust to cater for the new demand. By pumping you will get an increase in your milk production and in a week or two will be making more than enough to freeze (by which time you will have plenty to give your baby at the breast as well as to store)
Don't worry about your baby not getting enough milk at the breast, that won't happen if you are feeding him often and thus feeding your supply.  

[meiram]

When my little guy was that age, he often only needed one side, so I would pump from the side that he didn't use.  Sometimes, even if I thought he had drained both breasts, I would give it 10 or 15 minutes and then try to pump.  I wouldn't get much at first, but slowly but surely my milk increased.  

[Ecologic]

Thanks for the answers! I started pumping yesterday, both when he feeds and during his naps. I hope my milk increase soon!

[karla78]

the recommendation is to pump halfway in between feedings, at least thats what ive read and what the lactation specialist told me...good luck!