time interval

I want to write an expression that changes an objects' attribute every 25 frames.
for instance:
Every 25 frames blinn1.colorR has to change to a random value between 0 and 1.
If I would just use "blinn1.colorR = rand(0,1);"
it would change every frame. How do I let it change only once every 25 frames?
thnx

how about:

if( frame%25 == 0 ) blinn1.colorR = rand(0,1);