As a guide, let's look at
14 CFR ?23.397, "Limit Control Forces and Torques". It says the maximum loads on the flight control surfaces must never exceed what would resullt from the given pilot forces. It gives the maximum (limit load) pilot forces for a stick as 167 pounds for elevator input, and 67 pounds for aileron input.
Also,
14 CFR ?23.395, "Control System Loads", states that
" ... Pilot forces used for design need not exceed the maximum force prescribed in ?23.397(b)." And that
"Pilot forces used for design are assumed to act at the appropriate control grips or pads as they would in flight ..."
The control stick should be designed accordingly.
--> So David Paule's suggestion that the stick should support one's weight is not a bad approximation (as long as you are about 250 pounds)!
(The 250 pounds is the Ultimate Load which is equal to the Limit Load of 167 pounds times the 1.5 Factor of Safety usually applied.)
(Yes, I know we are Experimental and the FAA design CFRs don't apply, but ignore them at your own risk!)