When they give slightly to a gentle squeeze, they are ready to go. I like to buy a rock hard one (usually needs 4-5 days in counter), a relatively hard one (about 3 days) and one that gives just slightly for that day or the next.
If they all ripen at once, toss them in fridge. It will help them keep for a few days.
To ripen faster, store in Brown paper bag on counter.