It is good for 6 months. As of the beginning of 2019 you just pay for it after applying through Radex and they do it right there in migraciones. They will most probably do it anyway during your cita. The issue might arise if it takes longer than 6 months for them to process your paperwork AFTER the cita - they will then email you requesting you get a new one on your wn. Happened to someone on this forum this year - he had a cita in August 2019, but they didn't get to deal with it till February 2020, at which point they contacted him requesting he brings a fresh one...So then its good beyond 90 days? Because I did it in August, and my immigration appt is in December. And yes, I did a cita preferencial.