I often use PET bottles and a plastic carbonation cap when I bring my keg samples to various places. It all depends on how good the seal is. I often have to use pliers to tighten the carbonation cap tight enough. Just add some more CO2 if the bottle feels soft. I have kept PET bottles that way for several weeks, and sometimes even for a couple of months. Just make sure the temperature is quite low (less than 40 F). I have not had much success with the stainless steel carbonation caps. For some reason, on PET bottles, they seem to 'leak" much more than the plastic caps.
