Don't know where abouts you are but i would say it depends on the unit. In most cases where we are it is around 20 mins from page to on scene depending on where it is in the area and the priority of the call which remember is built up with for more traffic and traffic lights to and from the unit.
what annoys me is when i can get from home or work to the unit, get a truck to the scene and still would have had the job done before the the on scene service (i'm not trying to have a go at all firies, some are very good but others really need some practice in the basics), and the casualty wouldn't have been bounced around nearly so much or nearly slid sideways off the ladder.
Its funny that you get a tree down over road it will get handed over as soon as we rock up as they need to get back to cover their patch. However make that remove a window or something to get someone out of a second story they will be trying to stay as long as possible even if another service can do it is on scene with the tools to do it while they are waiting for a truck that is 15 mins away, or while waiting for that truck they'll put a stop on the SES truck that is already mobile and closer to the job.
what I'm trying to get at is that if you want metro people to keep volunteering (as the can't join the CFS / SAAS in most cases and how many roof jobs can you do) you can't keep wasting their time with stops and sending fire services P1 and SES P2 all the time, and stopping jobs where they aren't finished and the vollies can still contribute to the outcome. People get the sick of it don't respond and then when you want them for example when that wingfield fire was one we had a rescue of someone on torrens where stokes was needed and not a fire truck to be seen you may not have them.