It depends on your ability to chain. I have found that if you chain effectively It can only take 10-20 minutes to find one with a HA. Chaining is when you repeatedly uses the dexnav search to find Pokemon that are shiny, have perfect IVs ,HAs, etc. Once you get to the area spray a repel so you don't encounter Pokemon by walking in grass. Then search for you Pokemon with the dexnav. Every time you find an eevee and catch it or KO it you will increase you chain and you chances of finding the eevee with HAs. Leaving the area, encountering Pokemon through walking in grass, turning off the game, Fleeing from a battle, and battling trainers will break the chain and make you start over.
Hope I helped!