To even get a hidden ability, you need be online with your friend at least once.
The actual percentage of you running into one is unknown (some say like 50% others say 75%, someone even said 100%). But, it seems as if the chance of getting one increases while your friend is online. So if I was your friend and I had been online with you, the chance of you catching Phantump with its hidden ability is increased, but if I was done playing for the day, it'll take you awhile to find that HA Phantump.
Hope I helped, even though I couldn't tell you the exact percent.