I'm not aware of a set frequency listed in a standard, but in my last organisation, a multi-national with approx 400 manufacturing sites in Europe, it was group policy that every one was tested weekly and recorded. This was done during normal production, with each time a controlled stop being required, a different E-Stop was used until they had been all tested. The only exception was E-Stops that would cause waste product and these were tested at start-up on a Monday morning before material was fed into the machines.

It will depend on the design of the E-stop which in turn will depend on the level of hazard. Many E-stop buttons are dual circuit (2 sets of switch contacts) so if one contact becomes stuck on, the system will detect this as a fault. However if a single-circuit E-stop becomes stuck on, you have no way of knowing this until you press the button and nothing happens. These systems require very regular testing.