Maybe I'm one of the lucky ones, but my lo has been on a 4 hr EASY since he was about 4 or 5 months old. But, he did this himself. I didn't have to struggle, he just slowly stretched out his A time and his S time on his own. If you are just starting on an EASY, aim for 3 hours, but mostly, watch your los cues. See waht natural EASY your child adopts and work from there. He did start by spreading out his night feeds first. Sometimes it would be a 3 hr easy, others a 4... I just watched him to make sure it was what he needed.
Of course, during the growth spurts you have to accommodate.
And as for your lo wkaing only once at night, I think that's awesome. I'm not sure how your other children did, but that's great for a bf baby. Now if your lo wakes close to their normla wake time and are hungry, could you just give a small feed so that come their first morning feed they are genuinely hungry?
If you get the days worked out, then hopefully it will carry over to the night, but as Emma has said you may want to try the EASY forum as well.
HTH