We bought APs as well and got the voucher's as well which we exchanged for the real AP on the day of visit. Would do it again, aside from saving lots of money on entries, also save on purchases and food (if you buy the option). It doesn't matter how far in advance you buy it, they last until you first activate. You can have it for years if you need to - I had planned to do this, but things changed and we didn't need to, but originally I bought ours 18 months in advance (to avoid price rises and cash in on the good dollar at the time).