Should I be keeping track of EC and volunteering hours for my resume/college admissions, and if so, how?
For the Common App, you should have an idea of how many hours you spend per week on each activity. (College applications don’t ask about volunteer hours. Your ECs can involve community service, but they don’t have to.)
What if the weekly hour amount changes from week to week or seasonly?
Use an average. You don’t have to be exactly accurate, but you should be within the realm of reason; e.g. don’t report 2000 hours if you’ve logged 100.