Hello Peter,
I'm sure others more up to date on this will come to your rescue with perhaps a more informative answer, but I too believe the true answer may be as vague as you have already received. If this car as being used on say a 12 to 15k mileage per annum then the service light might come up at about 15 to 18k. However, at your lower mileage, it's unlikely that you will get to that mileage before the light does come on, since the system is logging the distances travelled, average speeds, time etc. , so indeed it is difficult to predict, but perhaps around 18 months from the start - ?
Personally, I believe you can trust VAG's technology to let you know when it's due, but it is that important to you then perhaps you can talk with your dealer and get this off the "longlife" service interval it is probably set on, and arrange to have it serviced say, annually.
Kind regards,
Gareth.