I attended orientation back in 2017 with Schneider in Phoenix and they had me doing the physical first thing in the morning. It was a very extensive physical to, they make you do trucking related tasks that are simulated and you have heart monitor on the end of your finger, if you register too high for your age, you're out!
One guy only got a 90 day card because he had a hernia, a very small one. Not many of these big megas will accept a physical unless its done by their people. When I started with Swift back in 2013, the accepted my DOT physical dated within 30 days of the orientation, but that was then.
Just to add some more recent experience, I had my first day of orientation today and they accepted my existing DOT medical card. It seems like it may depend on how long is left on it. A few students in the class did have to do the physical today but most didn't. I still have about a year and a half left on mine. I didn't ask specifically, but I think the ones that retook it had less than one year before expiration.