Simple expression for varying opacity over specific time

If you want an abrupt change in value, but not on every frame, you could use this expression.

After Effects 7
Useful things
Expressions

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Although the second line would normally create a new random value between 20 and 60 on each frame, the preceding posterizeTime(frames per second) method is lowering the frame rate of the resulting expression.

posterizeTime(2);
random(20,60);

If you want something that gradually changes over time, you could set the Opacity value to the middle of your desired value range (i.e halfway between 20-60 is 40) then use this expression. The first number is wiggles per second, the second number is the max amount the value will change.