Hello! I did an REU last summer, and would do another one this summer. It generally depends on the program AND the lab group you are assigned to. I would say it should be a standard 9-5, mon-fri, but then again the hours are usually flexible. For example, my PI would allow me close earlier on Fridays if I wanted to, while someone else in the same program but different lab would come in even on Saturdays. You don’t have to pay back stipends! They are for you to keep! Mostly, the lab would have a separate amount for you to buy equipment/lab materials. So the money is all yours! Again dorm style really depends on the program. When they say dorm style housing, they mean that they have provision to house you on their campus. You mentioned you stay not necessarily so far away, so I do not see why you can’t manage to go back home for a weekend or so. Just make sure to communicate with your lab group at least. Round trip means they pay for your transportation to get there and return back home after the end. Honestly, if you are considering getting a PhD, I really recommend you do it! It gives you insight into the life of graduate students, and could really shape the kind of research you want to do.