Is it better to do undergraduate research in the summer, or during the school year?
I think the main question is a false dichotomy. Most students don't have a choice between either doing research over the summer or doing it during the year. As long as it doesn't interfere with your grades or other activities, more research is always a good thing, no matter when it happens.

That said, in my experience you should simply have a conversation with your professor (or direct supervisor, who will often be a grad student or post-doc) about time commitment and expectations. No one expects you to do the same amount of work in 10-15 hrs/week that you might do in 30 hrs/week, and no one expects you to work full-time while taking a full course load. As with many things in life, just make sure to communicate about expectations.