I etch my steel in Hydrochloric acid. How long depends on the temperature of the acid. If the acid is new and I can heat it up during a warm summer day it can take 8-12 min, but if the acid is a bit old during the winter it can take 3-6 hours. 

Keep in mind that Hydrochloric acid is very corrosive and the fumes should note be inhaled. Always handle it with care outside, with both eye protection and breathing mask. Do not store it in an open jar near anything that is metal.