Where can I found out how many hours people work by the annual salary they earn?

IMHO, there is no formula (you can ask them, of course, if it is an
individual case): some positions require many more hours than the
official working-hours basis; some people in the same position put
more or less effort (in hours) into their work.

The formula you would need to use is:

Yearly Hours * Hourly Wage = Salary

If you only know the Salary, then you cannot solve the equation, but
you could graph it.  If you can figure out their hourly wage then you
can solve for a precise number of hours that they work (either weekly
or yearly... depending on what you're looking for).

On a side note, there are approximately 2000 work hours in a year (for
a 40 hours per week job which most people work) so the formula for
determining an hourly rate from a yearly rate is fairly simple:
For every $2000 yearly it is $1 hourly... $50,000 yearly = $25 hourly
(approximately)