The summer between MS1 and MS2 is generally free and you'll have time then. As a matter of fact, I've got a couple of students joining me for the summer to work on some projects.
During 2nd year and perhaps some of the slower 3rd year rotations, you also have time. Do you have a significant amount? Probably not, but unless you took a year off, were in the MD/PhD track or started on a project as an MS-0, its hard to produce anything of significance.
However, for most people, at most programs, in most specialties, its not necessary. Rad Onc, with a high percentage of applicants having PhDs, would be something I would think of as more necessary than Urology or General Surgery. So unless you are looking at PRS, I wouldn't worry about it (and even then there are people who get into PRS without research).