You'd be better off running 3 packs together if you want to postpone your bleed. It's fine to do that - the 'period' week is actually an artificial construct in that the scientists that designed the pill thought they'd better give women a 'period' so that they felt normal!
In theory you could take the pill indefinitely with no breaks at all, but you do increase the risk of spotting the longer you do it for.
My last pill free day was 34days ago. I had finihed my period as normal, then started a pack.
(Just incase it makes a difference, I'll explain...) Due to being on anti-biotics and forgetting its effect on the pill, I needed to take Levonelle once, and was advised by the pharmacist to run my next pack without a break (something to do with keeping up hormones).
I haven't ever accidentally missed a pill if that's what you mean though.
Okay, but antibiotics sort of counts as a pill-free day because it's a day when you don't believe it has worked.
In general I would say you can run together any number of pills that's more than 21 if you haven't had anything happen that's known to mess up the hormones (missed pill, antibiotics, etc). You don't actually need to have multiples of 21.
Could you go ask the pharmacist? Or phone NHS direct?