Longman Dictionary of Contemporary English
attention span
noun
EXAMPLES FROM OTHER ENTRIES
▪ Kids his age typically have a very short attention span.
EXAMPLES FROM CORPUS
▪ Dear Kidsday: My attention span in school is very, very short.
▪ Most puppies have a rather limited attention span, such is their enthusiasm for life.
▪ Try to increase your attention span and extend the length of time over which you can work effectively.
Wiktionary
attention span
n. The length of time, or span that one can pay attention, or concentrate on one topic without becoming distracted.
WordNet
attention span
n. the length of time you can concentrate on some idea or activity
Wikipedia
Attention span
Attention span is the amount of concentrated time one can spend on a task without becoming distracted. Most educators such as psychologists agree that the ability to focus attention on a task is crucial for the achievement of one's goals.
Usage examples of "attention span".
His teachers complained he had a short attention span, but what they meant was that he kept asking strange, difficult questions instead of sitting quietly and viewing his lesson programs.