Priming sugar in corny

I am going to rack from secondary into corny and allow to naturally carbonate in my keg instead of force carbing. I will purge O2 while adding a bit of CO2 pressure to ensure proper sealing. I am speculating this natural carbing will take about the same two or three weeks as if I were bottling.

Should I use the same 5 ounces of priming sugar solution in the corny as I would if bottling a 5 gallon batch?

You are correct in assuming it will take about the same amount of time to carb up as if it were in bottles at the same temperature. I find mine is pretty close after 2 weeks at about 68 degrees. But gets better over the next week on the gas. Not sure if this is from too little priming sugar or not enough time to get fully carbed up.

I use 1/2 the amount I would normally use to bottle a 5 gallon batch. Much less head space in a corny with 5 gallons of beer as opposed to the head space in 48 bottles, as a whole obviously. I may bump up my priming sugar amount to 2/3rd's next batch to see what happens. But I'd rather undercarb a bit then overcarb. If your undercarbed it will get to where it needs to be after being on the gas for a few days to a week.