Skin care: do we need to wait 30 minutes before going outside with physical sunscreen?

Yes. The sunscreen needs to be absorbed to be effective. In addition, it may not be truly waterproof so that it needs to be reapplied. Since it only helps against basal cell and squamous cell cancers, one needs sun protective clothing and sunglasses to decrease the risk of melanoma. If one reapplies sunscreen, it is helpful to leave the sun for 30 minutes with the second application.
Yes. Generally speaking, most maufacutrer's feel that it takes this amount of time for the product to adequatelly distribute itself over the skin surface.