Generally, as long as your shingles are laying flat, your roof shingles
are still in good condition.
The granules on the top surface of the shingles are intended to protect
the asphalt the shingles are made of from the UV light from the Sun. As
that asphalt deteriorates, not only do the shingles start to lose
granules, they start to curl into distorted shapes.
Here's a couple of roofs where you see that the shingles are just
starting to curl. Both of these roofs still have a good 5 years of life
in them, if not more.
If the shingles on the above two roofs aren't replaced, then the curling
will continue to get worse. When you see shingles that look like this:
Then the roof is at the end of it's life.
And, if the shingles still aren't replaced, you can end up with shingles
that look like this:
And, at that point, it's possible that some of the roof sheathing has
rotted under those curling shingles and the homeowner would incur
additional costs to have that rotted wood replaced.
If there are trees in your yard and the tree branches rub on your roof,
it's a good idea to prune those branches. Otherwise the branches can
brush the granules off the shingles and shorten the lifespan of those