mmm... I think the AdComs aren't really concerned at all with how many hours/ summers/ years of research you do as long as you get something tangible out of it. This may be an experience, a paper, whatever... As long as you can make a good case for it in your app.
Don't think that you can simply put it there for padding. If you do, it will be obvious, and people will likely be unimpressed. Do it if it interests you, and if if you like it. If you don't, put the time to better use and go deeper into something else you like. As mentioned above, many get into medschool without doing it. If you do it, it will only help you in the measure that you can talk eloquently about it having to do with your personal growth/interest in medicine.