Use this calculator to figure out how much sun time you need given your latitude/longitude, cloud cover, elevation, etc. https://fastrt.nilu.no/VitD.html

And keep in mind that using your skin to generate Vitamin D from the sun is a complex process that takes time. Exposure during the day typically results in Vitamin D being made available in the evening, and thorough washing can reduce yield.

You need the sun at > 40 degrees angle if IRC.

In the north this happens only during brief periods of time. It’s also usually during office hours...

Getting enough sun at some places or latitudes is almost impossible during the summer unless you work outside during the breaf period of time there actually is light outside. It's also cold.

A lamp is a minute or two and you're done. Sun requires 15+ mins and if you spend too much time you get sunburn. Using sunscreen works against you.