United miles expire, but it's also very easy to push that expiration date out for another year because almost any account activity resets the expiration date, even if it's a $10 purchase through their shopping portal, or 50 miles from a market research survey. If you or spouse also have some free time, United is also the most generous program for picking up miles outside flights and credit cards- I've managed 40K of them from the aforementioned market research surveys and using sites like MyPoints in the past year.

Come on down to the SPAM folder for more details in that regard.