Normally 1 hour, sometimes longer depending on what you want doing.
Had Infinity installed today. Installing the faceplate in the house was fairly quick as no new wiring was needed - maybe 10-15 minutes. Then work at the cabineta bit longer, probably about 30-40 minutes. Returned back to the house to make sure everything was working, another 5 minutes. All in all, about an hour.
With my install got a call to say the phone would be off for about 30 minutes. After that the guy showed up put a new faceplate on and showed me iplayer on my laptop. So like the other people in this thread have said about 1 hour total.
It took about an hour to install but it's going to take between 72 hours and 6 days for them to come back and repair the phone line they somehow broke. Not to mention the two hours you'll spend smacking your head against the brick wall that is their support.