Based on what you described, I would think that service duration and service length may be misunderstood. That is to say, they could be understood to refer to the time since they had joined your organisation rather than the amount of time that they had contributed. For example, a volunteer who had been around your organisation for 2 years but only contributed 10 hours of that time would look like they had contributed more than someone who had only been with the organisation for a few weeks but had already given 100 hours of their time.
I can think of a few alternate ways to express this, for example Hours of Service, Contributed Hours or Service Contribution. I'd personally prefer Hours of Service or Contributed Hours, depending on whether you want to emphasize the "Servitude" or the "Contribution".