How often should a person pray to stop them getting the coronavirus?

At least 8 hours a day, every day. Then of course when they get it they can blame someone else.

It really depends upon 'who' they are praying to, and 'their faith' in the 'whom'.

Joshua 24:15 But if serving the Lord seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your ancestors served beyond the Euphrates, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the Lord.”

As often as they'd like. Everyone will eventually die though. If not from this then some other way.

Once is enough, if it's sincere