Depending on the variety apples can be ready for harvest from July through to end October.
None of the early varieties are 'keepers'. This means they are left on the tree until ripe..then eaten quickly before they spoil.
Most of the late varieties are able to store, although any Apple if it's left on the tree to be fully ripe, will not keep for very long.
So if you have a late variety you can pick some of the crop before it's fully ripe and these will store in a cool dark place for eating later.
So.....you need to get to know what varieties you have got. Then a picking regime can be worked out.
After all that...damaged fruit, those with maggots in them etc, will often fall early.