From Hhonors Terms and Conditions:

"13. Members who do not earn points in any 12 month consecutive month period may be removed from the Program and, if so, are subject to forfeiture of all accumulated points. The account may be reactivated by the member taking one of the following actions:

a) stay at an HHonors hotel, thus reactivating membership; or
b) earn HHonors points through any third party source of HHonors point accumulation; or
c) request any Reward for which the Member is eligible;

account will then be closed at end of the 90-day period.
Failure to take action (a), (b) or (c) described above within the prescribed 90-day period will result in the closure of the Member's account and the forfeiture of all accumulated points. "

My wife failed to do any of the above one year, but we saved her points in the 90 day window.

From Continental OnePass Account Rules:

"Inactive Membership

If no mileage is deposited in your account for 18 consecutive months, commencing after June 1, 2001, your membership will be cancelled, and your miles will be forfeited. "

Note that redemption is not a valid activity.

From NWA General Membership Rules:

"WorldPerks miles have no expiration date. However, consistent with the general rules and regulations of the WorldPerks program, Northwest Airlines reserves the right to change the WorldPerks program at any time without notice, including imposition of expiration limits. If no mileage from any source is posted to a WorldPerks member's account for three consecutive years, the account is subject to termination, including forfeiture of all accrued mileage. "

I know noone who has had their account terminated.


From Marriott Rewards Terms and Conditions:

"An account may be closed at Marriott's discretion if no points are accrued during a 24-month period. All points in the account will be forfeited at that time."

Again, note that redemption does not appear to be a valid activity.
PS All of this information is easily discoverable at each specific website.