I've worked on a couple sets in the camera department for some commercials and had the chance to hang around a couple of major motion picture and television sets (knew the people who owned the property) and have not once seen a set medic. I know they are there, but as with any job on set, you do your job and stay out of the way when you're not.
Film, Television, and commercials sets are described as "hurry up and wait." When they're filming some dialogue, you'll probably be free to wander around or just sit back and watch any one of the monitors (though theres been a couple of times where the monitor wasn't where I wanted to be looking, but i digress). If they're shooting a stunt scene, you'll usually stand somewhere just behind the camera crew, just in case something goes wrong. if something does, you'll provide care to the injured hopefully to get them back to shooting again or until an ambulance arrives.
The days are long, because you may have the first call time (which on some sets has been 4:30AM) and you'll possibly have to stay until all the equipment is broken down and everyone is going home (which on the same set was 1AM, though I have seen 24+ hour shoots).
Nice sets will provide you with 3 meals a day and a break to eat those meals, and have a snack tent/table for you to freely feast on. On one set, the catering company walked around all night with food. They just kept walking around the set with trays of food and handing it to actors and crew as they wanted it. Another set they just let us leave to get food and would reimburse us for the cost.