There is a lot of ways that someone can interpret the saying "only bet what you can afford to lose" but people cannot really decide on how much is too much. Especially during high adrenaline events like betting.
They should set their limits beforehand. For me, they should never spend the equivalent of their salary in just one bet. You should only bet with your extra money not your entire paycheck of the month. Because if you lose, what will your plan be then? Some only takes 1%-2% of their salary but it depends on how big the salary is