Q:

"in 60 seconds or less" or "in 60 seconds or fewer"?

Tell me please which sentence is correct.

I want you to articulate your ideas in 60 seconds or less.
I want you to articulate your ideas in 60 seconds or fewer.

The word second is a countable noun, so theoretically it should be 60 second or fewer, but what I heard English native speakers say is 60 seconds or less.

A:

"60 seconds" is the amount of time, so by saying "60 seconds or less" you basically say "in less amount of time than 60 seconds."
According to Lexico, less is also used with numbers when they are on their own and with expressions of measurement or time, e.g.:

His weight fell from 18 stone to less than 12.
Their marriage lasted less than two years.
Heath Square is less than four miles away from Dublin city centre.

From an answer on English Language and Usage SE:

The general principle here is that less should be used when describing
  a continuous quantity and fewer should be used for a discrete
  quantity. This is not the same as countable v non-countable.
Time is a continuous quantity, so "5 minutes or less" is correct.
A purchase is a discrete quantity, so "10 items or fewer" is correct. 

