Dependent status is derived, so it ends when the principal's does, including when the maximum is reached. A spouse's employment authorisation ends at the same moment. Children's dependent status also ends at twenty-one independently, which can arrive well before the principal's ceiling.
Two ceilings, and they rarely coincide
Families settle in and start planning around school years and careers, both of which run longer than the classification does. The principal's ceiling is fixed by the cumulative maximum for his classification. Each child's ceiling is fixed by a birthday.
Neither moves, and they usually fall in different years, which means the household faces two separate transitions rather than one. A spouse working under employment authorisation that flows from dependent status is exposed to the first of them, and a spouse who has built a career locally should understand that the authorisation is derivative rather than her own. The planning response is to name both dates at the beginning and decide what happens at each: a change to student status for a child approaching twenty-one, and for the household as a whole a decision about whether to pursue an immigrant route, move to another classification, or return.
Each of those takes years, not months. Hypothetical example: a museum-exhibition company transfers its operations executive while her spouse has a freelance design practice and their child is finishing secondary school. The first review produces a family status calendar showing each admission end date, the child's age-out date, and the spouse's work-documentation needs.
The I-94 records settle the household's current period of stay; the child's birth record settles the age question. They should decide whether the spouse will continue work only after confirming the status evidence required by an employer or client. Problems arise when the family plans around the principal's approval notice but overlooks each dependent's documentation and later travel.