Just a little background before my actual question!

So I’ve been taking adderall on and off since my junior year of high school (since I was 16), and I’m currently 21. The most I’ve been prescribed was adderall 30mg IR for about 2 Years. I stopped taking it at some point and then started getting prescribed it again but at a lower dosage (20mg adderall IR). This past year been taking 15mg IR, but today I was prescribed 15mg adderall XR. I’m not sure why I was changed to XR, but I honestly think my psychiatrist accidentally wrote down XR instead or IR. I’m not sure if I should call back and ask him about it, I probably will! But for the time being:

I have a few questions about it:
(yes I know I could just ask my psychiatrist or a pharmacist, but people who actually take it have experience and an understanding about how it works.)

-How long does it take for it to take effect?
-How many hours does adderall 15mg XR last for?
-Is the XR come down any different (easier or worse) than IR?
-Should I switch back if I don’t need it to last as long?

Thank you for any helpful answers! (:

ps. I understand it effects people differently, but it’s more helpful than not knowing.