With regard to keeping track, I never really kept track of the hours - I thought this was a bit neurotic and I volunteered because I enjoyed it, not because I needed so many hours. I always see pre-meds looking at their watches/phones while volunteering to keep track of their "1 hour a week" because their world will implode if they go fifteen minutes over. Just get a good sense of how many hours a week you volunteer and then multiply that over the time of your experience to get the rough number of hours you volunteered.
Most likely, nobody is going to check that you volunteered the amount of hours you said and even if they do, many volunteer supervisors don't actually know the exact number for each volunteer so as long as it's in the right range, it's not a problem. If you say you volunteered for 1000 hours but have absolutely nothing to say about your experience, some people might get curious and your supervisor, while maybe unable to tell the difference between 200 and 250 hours, will definitely be able to tell the difference between 200 and 1000 hours.