You would most likely need at least 1.5 tons if it was a smaller width swale (12" - 16" ). For any more width, figure the sq. footage at 75 - 80 sq. ft. per ton.
We just installed 2 similar jobs with the same stone and averaged about 75 - 80 sq. ft. per ton. This was more than just a single layer of stone in the dry creek beds so you couldn't see the landscape fabric underneath. Give us a call at 704-238-1900 and let us know the exact length / width and we can help you, if needed.
Visions Landscape Supply & Design