The answer depends on your loyalty program. As of 2022, Delta, JetBlue, United and Southwest do not expire milesfor the life of your account. However, American Airlines miles expire after 24 months of account inactivity. Alaska Airline miles don’t expire, but your points will be inaccessible if your account is deactivated following two years of inactivity (until your reactivate your account, thankfully). Similarly, Frontier miles don’t expire but you must pay a $50 to $500 fee (based on your total expired miles) to reinstate your rewards if you don’t accrue any other points in six months. Fortunately, Frontier Airlines has decided to pause mileage expiration due to the pandemic but will alert accountholders within 90 days of when they plan to resume expiring miles.
Some loyalty programs might offer the chance to buy back miles that you've lost to expiration, but be prepared to pay more for them than the value of unexpired miles.