Id just like to add here being a guy that does trans flushes all day long..
A trans that has never (or you dont know) been flushed.. After 75k.. DONT FLUSH!!

Now.. jerZcavy4120 was completely right about the 50k trans flush intervals..
On.. Most GM cars and trucks..
Now.. That being said.. ALOT of Fords are 30k.. And if it hasnt been done by 55 or 60k.. Pan drops are advised...