A very close personal friend of mine said that he never checked with anyone who may know how many days early he could pick up his Rx, even though he had the chance because he felt it would look suspicious.
Instead, the first 2 months that his Rx was going to run out before his next appointment my friend called the doctors office 4-5 days early and picked the paper prescription up and filled it 3-4 days early. During the worst of the recent/ongoing Adderall
and Dexedrine shortage my friend said that he mentioned the difficulty he had finding a pharmacy with his medication in stock to his doctor during a visit, then when his next refill was something like 10-12 days away he called, explained the problem with the medications scarcity, and had no problem picking it up and then calling tons of pharmacies and having it filled the next day. This was a one-time thing though.
He's been prescribed to take dextroamphetamine daily for just over a year now and every month except for the first 2 when he had appointments before he needed a refill and the one month where he had his prescription filled around 10 days early he has had it filled between 6 to 2 days before his previous one was going to run out with no problems. Each persons experience will vary obviously.