@bubke
When I lived in the ashram in Germany I did intensive Pranayama sessions where our kind of Nadi Shodana, it is with retention of breath also, was done for up to 60 minutes.
Since it is calming and harmonizing you can do it for a long time, time is only limited by the amount of time the practitioner can sit comfortably.
Since it is the preparation for the energy awakening pranayamas of Kundalini Yoga, we needed to do it for quite a long time in those sessions before we could do those practices safely.