When/time to tour colleges?

We started second semester sophomore year. By the time my daughter applied to colleges first semster senior year we had been to 9 schools.
Junior year most high schools allow students a few excused absences to go on college visits.
Going on visits while the campus in in full session is best. During the school year, not during the summer, if you can..
My daughter was a dance and math major. We called the admissions office at least two weeks ahead of time. We set up meetings with admissions, the head of the math and dance departments, ask to sit in on a Dance class, ask for a tour of the campus and ate in the dining hall while there. At some colleges they set us up with a Dance major to eat lunch with. At one school they invited her to sit in on a math class.
Visit at least one large and one small college, one private and one public school. My daughter figured out fast a large state university was not for her.

By beginning of senior year you should know what colleges you have a chance at getting into. And have a firm list of 3-6 schools you will be applying to.

Any time during your school breaks is a good time. I especially recommend spring break because most universities will be in session and you will get to see students around campus and going to class. That will give you a good idea of what the atmosphere is like. During the summer break a lot of students take the summer off so there won't be much going on around campus.
Hope I helped and best of luck!

Any time during high school is a good time. But definitely junior year (11th grade) and senior year (12th grade) are the years you really want to start looking at colleges. Look for a time when your kids' school has a break, and go visit the colleges then.