I just bought a coffee porter extract kit to brew this weekend. This will be my 4th brew. This is first kit that doesn’t give me a ballpark on how long to ferment. I talked with the guy where I bought and he said likely just 10 days and just keep checking spg and when it hits the stated fg it is ready to keg. Is this the best way to know?
You could run a fast ferment test to determine the batches final gravity.
Wait until you see some airlock activity and pull a small sample from the fermenter. Let that go to completion at room temperature, shaking every time you pass it.
That’s the most full proof way to know it’s done fermenting. You would then reference the FFT gravity to your measurements to know when the beer is finished.
It’s highly dependent on gravity, pitching rate, strain, and temperature, but as a very general rule an ale taking more than 10 days to reach FG would indicate a problem.
One note though, you say the store rep said once it reaches projected FG it’s done. That may not be true, under the right circumstances, it could go lower and thus cause problems after time in bottle if you are bottling. The target FG is just that, a target. You won’t know it’s done until you get consistent matching gravities a couple days apart or it hits the same as your FFT. This last of what I said has already been stated above, I just wanted to clarify the part about the “target” to avoid potential bottle bombs if you bottle