There's no fixed answer to this because it depends on the volunteer organisation and, for some services on things like weather and geography. However, for most employees this is something they do outside their paid employment, in evenings and weekends.

If your employee is called out for emergency response during working hours, it is hard to say how long they will be away because it will depend on the scale of the incident.  Ask your employee more about this. For example, Scottish Mountain Rescue have produced a breakdown of the sorts of incidents they attend and the average time commitment for each.

No, it is fine for you to say that you need them to stay. They will be fully aware of their contractual obligations to you as their employer. We suggest you have this discussion in advance and agree what will work for your organisation and for them in their volunteer responder role.

All volunteer responders go through rigorous training and development processes relevant to their role. They will develop skills in a range of areas from physical and mental first aid to health and safety, team working to leadership, and they bring all these skills these back to the workplace.

This case study from a mountain rescue volunteer outlines some of the benefits to his employer.

This depends on their role but most volunteer responders undertake their training in the evenings and weekends.  They are adept at managing their responder role around their daily work and family commitments.

This is something to discuss with your employee but they will agree that they can't be "on call" in two roles.
They tell their volunteer  organisation when they are available to respond and they won't be expected to do so when they have on call responsibilities in their paid employment.

It very much depends on their responder role and the incident they're been called to attend. Ask your employee more about this. For example, Scottish Mountain Rescue have produced a breakdown of the sorts of incidents they attend and the average time commitment for each.

When they return from a call out they may have dealt with something relatively straightforward or attended something potentially traumatic, like a cardiac arrest or a serious or fatal accident.

It is helpful to remember this because they may return to work in different mindset to when they left 2 hours ago.

It is your employee’s responsibility to make sure they’re physically and mentally fit to return to their workplace and they will become adept at managing this transition. However, we recommend you talk to them about how you and they can best deal with this and then let other workmates know. They might appreciate a cuppa and a quick “You OK?”  but are unlikely to want (or be able) to discuss the incident they’ve attended. Take your lead from them.

It is normal for people to be a bit preoccupied after they’ve attended an upsetting job so don’t be surprised if they’re a bit quieter for a few days, or perhaps a week or two. You can understand more about reactions to trauma and stress on this website.

If you’re worried about them, ask how they are. It may or may not be something to do with their responder role, but they will appreciate you asking. Have a look at the advice on the Good management and Leadership page.  

Most volunteer emergency responder organisations have insurance in place to cover their volunteers in the event of sickness or injury. Ask your employee more about this.

If your company employs less than 250 people you can access free and confidential health advice and support through Working Health Services Scotland (see below).

Working Health Services Scotland (WHSS) provides free and confidential advice and health support to the self-employed and individuals employed by companies with less than 250 employees. WHSS provides employees with access to several free, confidential services and aims to refer to services quickly thus helping to prevent further deterioration of health problems. The Case Manager liaises with GP’s and other relevant professionals to ensure effective outcomes. Phone 0800 019 2211 or visit this website to find out more.