again, i am biased, but 500 shots a day seems like a piece of cake (if that's the only thing on the agenda). but for someone that has not touched a ball in years or is completely out of shape, then maybe it would take a 2-3 weeks to get to that point. doing an actual 3-point competition using racks of balls is really very tiring trying to get all the shots off before the buzzer, but if you were to treat each 3 like a free throw and take 20 seconds between shots that is under 3 hours of time to get it done.
If the hypothetical is that rando person keeps their 9-5, can only sneak off a 1-2 hours a day to practice, then yeah. if this person can suspend their life to train, maybe even use a coach, then I think 50% unguarded (from the corner) is achievable. the mechanical act of shooting a basketball by yourself with no defense is not that hard to learn and improve on (Rick Barnes coached teams shooting their FTs excluded)
I don't think there would be that much added pressure if you got to 7 in a row with 250 days to go. if you are down to the last week or so then yeah, most would probably channel their inner Brett Maher under the pressure.