According to the Worker Adjustment Retraining Notification Act,

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most large employers must provide employees with at least $90$ days severance pay when laying off more than $50\%$ of their workforce.

most large employers must give $30$ days notice to employees if they layoff more than one third of their workforce.

most large employers must give $60$ days notice if they layoff a third or more of their workforce.

all employers must provide employees with at least $60$ days advance notice when laying off $25\%$ or more of their workforce.