Well, you need Construction Brigades to take use of alien ruins. So I usually build about 20 of them or so. But my PDCs are also really small as I rather build a bunch of different PDC classes on each colony, so the construction times are not too long. However, with a PDC Hangar, that's not really an option, meaning that Rich is correct - your only practical option is to use construction factories as otherwise it might take ten years or more.