For me, it was the right time when I couldn't handle it any more. I had tried giving him a big bottle of formula at dreamfeed, and it made no difference, and by 8 months he was eating really well, gaining weight. So I decided that, even if he was waking hungry, we needed to break that habitual hunger. For example if you were to go and raid the fridge at 1am for a few nights, soon you would start to get hungry at that time. At 6 months I still felt like he was having 'tastes' of food rather than substantial meals, so I kept feeding at NWs at that point.
So I accepted that yes he might be hungry the first night, but it wouldn't harm him, hunger passes, and he would make up for it in the morning. It was actually much less of a struggle than I had imagined it would be, once I made up my mind to never feed at night. If I sometimes fed and other times tried to settle without, it was a much bigger battle.
PUPD works great because you don't have to leave them to cry, you help them work it out. But, it doesn't work if there are any other props present ie a dummy/paci.