Pumping

How soon should I start pumping after baby is born? I'm EBF and today is day 4 I am beyond thrilled my milk has started to come in, I can hear baby gulping as he feeds and I am leaking alot. I want to start a good stash, but i dont know when to start. We dont use bottles yet, or binky's not planning to until, after we establish a great lach and good supply. I feed on demand every 2 hours for 15mins or sometimes a little longer depending on baby- Any suggestions or advice on this?