Is strengthening your application your primary reason for wanting to pursue research?
Either way, I wouldn't recommend going into research with a set (or minimum) amount of time in mind. Instead, I'd recommend committing yourself to do it for at least 2 to 3 months, so you have a chance to really get into it. At that point, evaluate how you like it and whether or not you'd like to continue.
Research can be an incredible experience, but it's not for everyone, and it can be quite time consuming. If you get into it, but don't really enjoy it, then I think you should reinvest the time somewhere else, like into shadowing, volunteering, or following some other hobby.
My logic here is that the schools who value research most heavily are research-oriented schools. If you find you don't enjoy research, then you might not enjoy going to a research-oriented school, thus lowering the value of research in your record.