There are the Southwest’s of the world with expiring credits every 24 months (used to be 12 months, but there were no capacity controls on awards — so in that case the tradeoff was well worth it).

But then there are the major carriers, where the rough standard on mileage expiration is that you need account activity every 36 months, otherwise your miles don’t expire.

But Air Canada is introducing new rules, as explained on Flyertalk but an official from Aeroplan:

Change #1: Effective January 1, 2007, Aeroplan will begin date-stamping Aeroplan Miles. This means that Aeroplan Miles issued on or after January 1, 2007, will have a date-stamp of 7 years (84-months) from the month the mile is accumulated. Miles in an account that exceed 84-months without being redeemed will be automatically deducted from members’ balances on a monthly basis. All Aeroplan Miles issued prior to January 1, 2007 will be date-stamped as December 31, 2006, resulting in an end date of December 31, 2013.

…

Change #2: Effective July 1, 2007, Aeroplan will change the terms of its mileage expiry policy to require members to have transacted with the program – either one accumulation or one redemption – once in a consecutive 12-month period. This means that, as of July 1, 2007 and onwards from this date, a member must have transacted once with the program in the 12 months prior to avoid expiry of all miles in the account.

Twelve months of inactivity closes an account — that’s truly at the extreme end of what any airline program imposes (technically Starwood’s rules impose the same thing on the hotel side, even stricter since only hotel or credit card earning counts, but in practice it isn’t enforced). And even if you keep your account active your miles will expire if you don’t use them in seven years. These rules make playing with Aeroplan on an truly occasional basis rather difficult.