I did a year and a half of computational and sensory neuroscience at my ugrad (full-time summer, part-time during semesters). When I graduated I did a year of full-time research in molecular neuroscience at a nearby institution as a tech.
I guess I'm not applying now, but maybe it's useful info to someone
Basically nothing during undergrad (one very short summer internship); then one year full-time as a tech (cancer reserach) while applying. When I wrote my secondary essays I had only about 4 months worth of real research experience. Did just fine in the application process (deciding between Vandy and UChicago)
Moral of the story: you don't necessarily need publications or years of experience, just good recs from PIs and a reasonable understanding of what you did (for essays/interviews).
3 months (3 hrs/wk) - physics research on light emitting diodes (obviously, didn't enjoy it, didn't stay long)
3 months (summer internship -- 40 hrs/wk) - worked in a museum in a fish taxonomy lab (not applicable to the kind of research I'm interested in, but it was something fun and different to do).
Currently (will be a total of 2.5 years (semesters only, not summers) when I apply -- 5-15 hrs/wk) - trafficking of neural stem cells, including work on the cellular level and animal care/breeding/dissection. Mostly been functioning as an assistant for the past year, but will start an independent project & write a thesis on it over the next year.
This summer (full time) - research internship at a medical school, have not chosen a specific lab yet, hoping for infectious disease work
Also, will be taking a year off while applying, hopefully working full time in a lab.
When I applied, I had 2 academic years and 3 summers of research, but one of those summers was worthless (e.g. washing glassware, running a few gels, and reading CNN). I would underscore what ManchotPi and JD428 said about quality being more important than quantity.
I'd done three summers of research (inorganic chem, neuroscience, then immunology--so you don't have to have continuity at all!), and I was working on the third project (full time) through the fall as my senior thesis. Somehow it'd never occurred to me to do research during the school year other than my thesis...
And to add no original advice whatsoever, I'd say knowing your work is more important than having done a ton. Also, though, it's not only knowing your research--being able to explain how you contributed/thought through some of the problems/figured out how to analyze some of the data yourself is probably key.
Full-time two summers in completely different labs (one was plant genetics, the other mouse). One year part-time thesis research on leukemia. Two months into a year off/application year of retrovirus research at time of interviews.