VO2 max - How often run outside for accurate daily suggestions?

Hi, I run and ride exclusively indoors and I’m not planning to change this, although I might go for a sporadic outside run if I have to just to get a VO2 max measurement.

Can you advise how often I should do this just to keep the daily suggestions accurate?

Would be really appreciated if threadmill speed and distance would be accepted as well. I don’t think that they are going to be less accurate as outside since there is no weather and traffic involved.

Also, since I run on the treadmill every workout it would be much more consistent to have tests and suggestions based on the same setup. Maybe just don’t call it VO2max but something like ‘indoor running fitness’ (you can no doubt find a better description)