Internships may be done at any time, but the sooner one gets started the better. Summer internships are usually full time, whereas fall or spring internships can be full time or as little as 8-10 hours a week during the school year. Opportunities are always available. Positions can be found worldwide. While some internships may be available only to juniors and seniors, there are also plenty of opportunities for freshmen and sophomores.
Internships are truly a case where the more the better.
The student will gain skills and networking contacts that can often result in permanent positions. There are many forms of compensation, and money is only one of them. Internships can be expected to pay off long term. The process of finding an internship is quite similar to finding a full time position. It takes considerable time.