I use to use a 15 gal megapot with a 50 ft immersion chiller - I have since upgraded to a 20 gal Boilermaker. I could chill 10 gal of boiling wort to 70 degrees in less than 15 minutes with 50 degree water. The top of the chiller stuck out of the wort since the pot is a bit wider than most 15 gal pots. I suspect it would take another 5 to 10 minutes with a 25ft chiller. Just a guess.
Jumbo82 is correct. I have a 15 gallon pot and a 25' immersion chiller. It takes approximately 20 - 25 minutes for it to cool 5.5 gallons of finished wort from boiling to 65F. I use tap water to start and switch to recirculated ice water at about 100F.
Nice handy work Mario. I just made a 50' 3/8" chiller last night! Hell Ya! I made mine to resemble the rib cage design on another thread in here somewhere. I didn't have anything on hand to have that bid of a radius. Is that about 11-12" in diameter?