Generally the ideal time in VR is between 10-15 minutes. We would generally recommend on a first implementation not purchasing too many, and growing out the hardware as implementations increase/scale.

2 VR headsets can run a workshop of 30 people in a few hours.
More than 5 VR headsets, and you will need more than one technical facilitator to manage the headsets on the day.

Doing the math

Typically what needs to be considered is the amount of learners in a workshop session. For example, if in an initial implemenation there are 20 participants in each workshop in a given Workshop/session rollout, a suggested ideal amount of VR headsets is 5. In this scenario learners could rotate the headsets between users to get all 20 users to have the experience within1 hour before a workshop/session begins.