$time = `currentTime -q`;
pointLightShape1.intensity = rand(#,#)* $time/$time;
go to the light intensity> right click > create new expression> paste into bottom field> click create.
when using this obviously you need to change the light type, and replace # with whatever you intensity values are going to be.
The problem is that rand is evaluated per-frame, so it will of course create a flickering effect. As Kari has shown above, using the noise function will create a much smoother transition in-between frames.
Imagination is more important than knowledge.