Night intervals!

My baby is five weeks old. I'd been waking him up every three hours at night to nurse. A week ago I got the okay from a lactation consultant to do one four hour interval per night. Yay, slightly more sleep!

Now we do one four hour interval (after which I wake him), then one three hour (after which I wake him), and then one two hour (at which point he wakes me but not from hunger).

I'm wondering when I can safely lengthen my second night interval to four hours, too. I'm not so worried about him -- he eats well and is 90th percentile for weight. But I'd hate to mess with my supply. Five weeks postpartum, is it safe to do more than one four hour interval? Or at how many weeks can I?

Thanks!