I've flushed the ATF in the transmissions of four of my 240's using IPD's kit (their hose). To do this, put the hose on the radiator's output fitting, and let the engine idle for a time as you fill up about 2 quarts of waste fluid, then stop the engine, refill two fresh quarts down the dipstick tube, then run the engine again,... and on.

So, watching my containers fill up, although I didn't time it with a stop watch, my recollection of my impression is that it takes about 1 minute to fill a half gallon (2 quarts).

Just remember, though, that this was at idle -- I don't know how a higher rpm will effect the fluid's flow rate.

I was unable to find an ipd in-line filter.... could you please direct me to the location?
In general, return line filters require careful consideration. They must have bypass valves in the housings, (essentially a "partial-flow" type of arrangement as opposed to a "full flow") otherwise a restricted filter will result in a pressurized oil return line, including the oil cooler (which is normally designed to run at full flow for effective operation), as well as causing fluid starvation to the transmission.
I've seldom heard of any "real world" complaints about the quality of the internal filters in modern automatic transmissions. You can find plenty of advice on adding an oil cooler to the transmission line, an idea that seems to have a great deal of support, and with which I happen to agree, but an extra filter? Perhaps others feel differently, but I'd question the advisability of introducing any element which might restrict the flow of fluid, particularly in a well documented, and well known piece of machinery.

Marty
'96 855T