Weakness can be defined as either not being able to complete a task one time or fatiguing quicker than normal while completing a task.
Weakness will precede atrophy with ALS. The weakness is due to denervation as is atrophy. However, as soon as denervation occurs weakness is immediate whereas atrophy will take some time after denervation. If you think you have atrophy and you don't have weakness, then that atrophy is not due to ALS denervation.
As far as the timeline between denervation and atrophy and the rate of atrophy with ALS: that is something in the literature I am still in search of and have been for some time. There are people on this forum that can comment on that because of experience.