It says "a fraction (of a second) too long."
If you didn't look at him for 1 second, he may not notice it, but if you didn't look at him for 1.1 seconds, he will notice. That extra fraction of a second makes the time too long.
A period of time may be insufficiently long, or the appropriate length, or excessively long:
Insufficiently long: The bread is not very good, because I did not bake it long enough.
Insufficiently long: One day is not long enough to paint this whole house.
Excessively long: Because of the traffic, our trip to the airport took too long: by the time we got there, our flight had left.
Excessively long: The fugitive stayed at his mother's house for a day too long, and as a result he was captured there by the police.