How long does the typical fever last?

For context my son is a full term NICU baby. Now 2 and in daycare. Wednesday spiked a fever and hasn’t been back to daycare since. He’s been in pretty bad shape with high fevers and persistent cough. I haven’t taken him to the doctor because I feel he’s most likely got a virus and there’s nothing they will give him to help. When ur toddlers get sick how long do their fevers last? I want to think out little guy is just the regular post covid baby. But I also sometimes wonder if there might be something else to it? Don’t want to over worry but can’t help it.