Hi Suzi - the muliy entry one is valid for 6m. Before I got mine we used to get $ and pay as it saves the arguments at the airport. Some will try and say they cost £10 each and some £15. They are clearly marked as 15 USD. If you only have sterling give them a tenner each and insist on change but again this can be a nightmare as they will say they have no change!! I just find it quicker and no hassle paying in dollars. Dont forget if kids are still in your passport you dont pay for them - it is per passport and not per person that is in the passport.