takes about 5 minutes to actually do it – yes probably
but i bet it takes half an hour to get the bike all set up in the stand, sort the tools out, check the work needing done, take the headset out the old frame, get the old frame out the way, stick the new frame in, prep the new frame, put the headset it, clean it all up a bit, find somewhere to stick both the frames, check you have done all the work, sign it off, have a cup of tea.
jesus! you havent sprung this job on your nan! you've taken it to a proper bike shop. if they have to spend time 'sorting out tools' and finding somewhere to put the frames then they need to sort their sh1t out…
ha ha
just seen the cup of tea bit… might be right there. maybe its a 45 min job.