There's no bigger sin than a wasted mile.
Most American frequent flyer programs are quite gracious in allowing passengers to get credit for previous flights, even before they joined. For most airlines, six months seem to be the average grace period. There's one weird exception: Frontier lets members request credit up to 12 months before they joined, though the policy for current members is much shorter (180 days).
MileCards also covers some foreign airline frequent flyer program policies but some of the info already seems to be outdated on North American programs.
Below is a chart on how long a passenger has for retroactively requesting flight credit—the demarcation is the date of enrollment. For information that wasn't available online, each frequent flyer program was contacted directly.
|Airline||After Enrolling||Before Enrolling||Notes|
|Alaska||12 months||6 months||Online requests for Alaska Airlines flights must be received within 11 months of travel. Alaska Airlines flights within 12 months may be requested by contacting Customer Care. Partners generally require that credit be requested within 6-9 months of the partner activity.|
|American||12 months||6 months||Request for flight credit before enrollment must be submitted over the phone.|
|Delta||9 months||30 days|
|Frontier||180 days||12 months|
|Hawaiian||120 days||120 days|
|JetBlue||12 months||90 days||Partner credit must be requested within specific time frames:
|Southwest||12 months||12 months|
|Spirit||30 days||30 days|
|United||12 months||30 days*||*For a $50 USD fee, flight credit before enrollment can be extended to 6 months.|
|Virgin America||12 months||30 days|