I don't know the specifics about the part-time option for this program, but usually they are twice as long as whatever the normal residency is - so for peds, obviously 6 years.
The lame thing about them, in my opinion, is that the ones I know about are not part-time as in more reasonable hours, but are usually set up as an every-other-month thing. So for most people who have a reason such as a child to want a part-time residency, this is just a logistical nightmare. Every-other-month daycare does not exist. And if you wanted to do it to make something like breastfeeding easier, obviously this would not really help. I understand that in terms of scheduling people this might be the only way to do it, but I was kind of disappointed when I learned more about how they actually work.