Welcome to FlyerTalk, ukheather.

Most of the US-based SkyTeam programs have moved to shorter expiration times over recent years. (I'll let one of the experts speak to other programs.) However, if you have some type of activity every 18 months or so, your miles will not expire.

How often do you travel to the US? If you are making a couple of trips a year, expiration probably should not be your main concern. If you only fly every couple of years, there are other ways to generate activity in your account (hotel/dining/retail partnerships, etc).