I switched to pre-vet mid second semester of my junior year. I've been essentially getting 20 hours a week of vet related stuff on the side while overloading on classes, as well as leaving my weekends fairly open so I can see my long distance boyfriend every few weeks. I'm about ready to pull my hair out, but I tell myself that there are other people who had it worse and pulled off a better GPA than I am currently achieving... so it gives me encouragement!
Like others said, just ask around. Once you're "in" somewhere, you can ask fellow workers/volunteers about other opportunities, and make a nice little network. Connections from shadowing at a vet clinic 10 hours a week last summer led to suggestions of farm volunteering (there's almost no LA stuff near where my parents live so that was great), as well as a list of mixed animal / hospitals that I should check out.
Another example... I've been volunteering at a large (and awesome!) animal shelter, and now that people know who I am, I'm getting enlisted to help with random odd jobs. Next semester I'll be a tech for the shelter animals, after just starting out with dog walking and data entry 🙂
Something else that would be good to look into is research opportunities at your school. I've been able to get hundreds of hours of really great experience with this.
During my year off after I graduate (and apply! AHH!) I have a likely position lined up with the shelter.
In terms of how many hours to aim for, check out various vet schools' websites. Also, email the adcoms as they can give you advice on what will strengthen your application the most.
