There are more advantages to a fast chill than just to stop DMS production. Cold break for chill haze reduction, immediate pitching, lower risk of infection to name a couple.
As far as your times. Faster is always better. Best I've done is about 30 minutes to 20°c/68°f pitching temp. But that's using whirlpool and iced water.
Boil to 100°F happens in the first 10min for me. Last bit is the hardest.
Here's a Couple Tips
- Chill from top down (cold water into top coils)
- Ice You can't chill below cooling water temp and ambient temps. Ice chill water helps a lot.
- Slow flow Ideally your exit water out of the chiller should be 'close' to the temp of the wort any faster is just wasting water. How much flow you can achieve and be within 10-20% of wort temp, gauges your chiller effeciency.
- Seal it When you start to chill, cover your pot with lid and foil to prevent infection.
- Movement Whirlpool & pump is best. Alternatively you can move the chiller, up down side to side without breaching your foil seal to get more chiller contact with hot wort.
- Inline Chiller plate chiller or counterflow chillers are game changers. And can be used in conjunction with immersion chillers. When done right you get pitch temp wort out the end instantly.
Ice chiller example from Zymurgy Magazine.