I have very limited experience with fermenting in a corny keg. My second beer fermenting that way has been in the corny fermenting for 11 days as of right now.
1. Some people ferment, dry hop, and serve all from the same keg.
2. Some ferment and dry hop in the keg, but then move the beer to a serving keg to get it off the yeast, trub, hops, whatever.
3. And some use three kegs. Ferment in keg1, dry hop in keg2, and serve in keg3.
There may even be other combinations or different ways to do this.
Here's what I did.....
My first one, a pale ale, Verdant yeast, OG 1.054. I did option #3. I transferred about 4.0 - 4.5 gallons from my boil kettle. Fermentation and CO2 transfers went fine, no issues other than hop aroma and flavor wasn't as pronounced as I'd hoped. I may have pushed the transfer into the serving keg too fast. I wonder if a slower transfer would have kept the flavor more intact, because the aroma coming off the blowoff during transfer was amazing.
My current beer fermenting is a Raging Irish Red, WLP001, OG 1.058 with no dry hops, option 2 above. I pushed this one a little further by transferring around 4.75 gallons, actually a little more with the yeast starter. I have a jumper running from keg1 gas-in over to keg2 beer-out and on day 2 or 3 I could see krausen moving over to keg2, despite adding 8-10 drops of Fermcap S to keg1 at the start of fermentation. After a day or so it stopped. I'm also using a spunding valve on this one. It's set to 10 psi.
As I said, limited experience. I'm learning as I go.