How often do you change the tranny filter on your 4L80?

 

I didn't have my 80e long enough... but I did replace my filter after break in. They're cheap insurance.

Why do you ask?

 

The filter isn't intended to catch the smallest contamination. It works more like a "screen". Only really the bigger stuff that could bind a valve is blocked. That said, in a healthy trans the fluid fouls before the filter does. Changing both at the same interval is fine, in my opinion.
I've always imagined an external 1 micron spin-on filter would keep the fluid clean but it would have to have a low-pressure bypass mechanism so it doesn't impede flow if it starts to clog up. Would be hard to monitor such an arrangement...