It depends on the variety of apple guest. Some ripen early and don't keep, and have to be eaten soon after picking. When a fruit is ready to be picked, it may fall off if it is windy or if it has had a visit from one of the many pests. You can test for ripeness by holding the apple and moving it slightly upwards. If it easily detaches it is probably ripe. Some apples keep well on the tree too. They stay attached for ages and taste nicer after a bit of frost. I have one tree that does that. I wish they were all like that. I don't know its name.