Covering for a coworker once is kindness. Covering for them every shift is paying their rent with your reputation.

Helping someone out of an occasional hole is ordinary decency and costs almost nothing. Doing it repeatedly changes the arrangement: their performance depends on your effort, your name is attached to work you never agreed to own, and nobody who could fix the problem finds out. Kindness has turned into subsidy.

Decide on a number and hold to it, without announcing a policy. When it recurs, say once and privately that you cannot keep doing it, which is fairer to them than quiet resentment. You are not responsible for a situation you were only asked to conceal.