There are several variables in coil life. One of them is the juice. Thicker juices, high VG juices, and very sweet juices tend to gunk them up faster. But even then, a good clean and dry burn, usually with also replacing the flavor wicks, generally restores them to like new, and can be done many times before actual coil failure.
I tend to clean about every 6 to 9 mls, and take off the chimney and flavor wicks then, to inspect the coil. If it looks gunked, I'll dry burn, otherwise just put in new flavor wicking, replace the chimney and call it good. (Cotton yarn, twine, or pulled out cotton balls works fine.)
Barring destruction by a tank cracking juice, or other mechanical problem, the tube will last a long time. Ever? Sure, stuff happens. Be prepared, have backups for everything, and backups for those.