Main drain very slow how long should i let sulfuric acid sit before flushing pipe?

when I used sulfuric acid to clear my drains i poured a very small amount in at first as a test because there was already a caustic drain opener in there that didn't clear the drain.I let the acid set awhile no problems then I kept adding a little more at a time until there was enough to clear the drain. I didn't have any problems and the plug was cleared .Acid is what I will use should the need arise in the future.let it set 15-20 minutes then flush