I don't know about Pekins from experience ( I had them decades ago and don't recall when they started laying), but I can tell you my little runners started around 16 weeks to 20 weeks (I have eleven, and it took a couple of months for everyone to begin laying).
I have read that larger duck breeds take a few to several weeks longer to begin, so I would think, all things being equal, that you would be looking for eggs in another one to two months.
A clue for me was that my runners started acting out mating behavior about a month before the first one started laying.