The temperature at the vents will fluctuate, starting higher and then cooling down to the minimum possible temperature based on many variables such as pressure of refrigerant, recirculation versus drawing in outside air, number of persons in vehicle, starting temperature and time run, external temperature, etc. Adding refrigerant without testing the system first is not advisable for reasons such as a leak may be present, and adding it could continue the release into the atmosphere and lead to early breakdown of the compressor since the refrigerant also contains the lubrication for the system. If you are drawing air and moisture from outside into your system, we will probably be reading about how your compressor broke down way to early and that today's systems are junk.
For OP here, make sure vents are pointed up since cold air falls, limit open doors and windows, keep lower panel of sunroof closed if you have one, and maybe tinting of windows. Good luck.