Hmmm...interesting idea, we have 16 hams and use a little under a bag a week now.
Say we use 4 bags a month that's 4 x 850g = 3400g a month. That means a 5kg bag would last us not even 2 months - I'd say it would probably keep fresh for that long - especially if stored in an airtight bin. Only thing I'd worry about is the amount of crushed dregs at the bottom of the big bag
Where do you get the 5kg bags from?
Incidentally I was in the petshop today and Harry now comes in a new design bag! I get mine from Wilko's though - they've still got the old style bags.....