About 40-50k. Heck I've always used the trans repair in any vehicle over 120k even BMW's. it helps prolong the life. I used to be a mechanic part time when I was a fireman before the railroad and the only thing I refuse to rebuild myself is a dang transmission

 

But I have only used it in automatic transmissions and have always had great luck with it. I used it in my work car (06 focus) right now. IMHO I think it helps the shifting and prolongs the longevity of an older automatic transmission. At times, I'll use a can of sea foam trans cleaner before I change the trans fluid. I put it in about 200 miles before the trans service and that is all based on the color and smell of the fluid.

 

This is the Lucas product I was referring to. You can get it at some Walmart and just about every auto parts store

 

Just my 2 cents. My 04 disco arrived new to me with 165,000 on the odometer. In pretty decent shape considering her age but with a hard shift between the first three gears and a nice "clunk" when shifting into reverse.

One of the first things I did was the entire high mileage service list, including draining about 5 qts of ATF. I dropped the pan, replacing the filter, seal and all pan bolts. The drained fluid wasn't burnt and didn't look too bad but it wasn't red either. What I failed to do was to add any Lucas to the fluid refill and the first fluid change did nothing to alleviate my hard shifts.

I lived with it for the next 11 months and 20,000 miles. Having read this forum incessantly for the past year I had already decided to change my tranny fluid at 20,000 mile intervals. So this past weekend I drained about 5 quarts from the tranny and the first thing I put back in was a bottle of Lucas transmission fix. Took me almost 40 minutes of struggle using a normally wonderful hand pump in the 30 something degree Atlanta weather to get it in there but once I added the rest of the ATF fluid (Valvoline MaxLife) and took it for a test spin my hard shifting has disappeared. FYI the Lucas is thick under the best of conditions but in cold temps it's like dealing with a super thick molasses! If I did it again in the cold I'd preheat the bottle in a pan of water on the stove like you do with a baby bottle.

But it was worth the struggle! Four days of driving and almost 300 miles later it still shifts as smooth as I could ask. So if you are having some shifting issues I'd highly recommend the Lucas tranny additive as a first step. Well worth the $ in my opinion.

 

Glad to hear this from a fellow Georgian! Im doing this on the weekend.
Cheers!