It actually will depend on the aircraft design, as that influences the purpose of the stick shaker. If the shaker is purely providing stall warning (the usual function) then the current FAA regulation, § 25.207 - Stall warning states:
§ 25.207 Stall warning.
(a) Stall warning with sufficient margin to prevent inadvertent stalling with the flaps and landing gear in any normal position must be clear and distinctive to the pilot in straight and turning flight.
(c) When the speed is reduced at rates not exceeding one knot per second, stall warning must begin, in each normal configuration, at a speed, VSW, exceeding the speed at which the stall is identified in accordance with §25.201(d) by not less than five knots or five percent CAS, whichever is greater. Once initiated, stall warning must continue until the angle of attack is reduced to approximately that at which stall warning began.
(d) In addition to the requirement of paragraph (c) of this section, when the speed is reduced at rates not exceeding one knot per second, in straight flight with engines idling and at the center-of-gravity position specified in §25.103(b)(5), VSW, in each normal configuration, must exceed VSRby not less than three knots or three percent CAS, whichever is greater.
So, as you can see, there's scope for a fair bit of variation, depending on how your stall speed is defined. The very general rule is 5% or 5 knots, but it's only a general rule.