Is there a general rule for ripening fruits after you buy them?
Every fruit differs when it comes to ripening. For example:
Watermelon and grapes should be harvested at full maturity since they do not further ripen after being removed from the vine.
Cantaloupe ripens after harvest. Ripen on kitchen counter at room temperature; refrigerate once cut. Note that not every variety of cantaloupe is soft when ripe. Whether you scoop the seeds out of the half you are not using is just a matter of personal preference – they don’t preserve the fruit.
Peaches and bananas will continue to ripen and increase in sugar content. Ripen at room temperature or faster in a brown paper bag.