When the stats are considered stale is subjective - there is no clear cut way to say they are stale. Generally you can look at how many rows were modified since the statistics were last changed and then you decide from there. Most of the scripts will look at the number of modified rows, last time stats changed to get an idea but a lot of times it's the query plans and the estimated row counts that tell you when stats need to be updated. This article has further explanation and an example script:
How to Find Outdated Statistics?
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