I've only had it be successful once. You'll know when it is, believe me. Your Sim will cough and clutch their throat, then fall to the ground. Then they will disappear into the ground for a minute and reemerge like the other zombies. The only interactions they can do with other Sims are several different "Aarrgghh" and "Braaiinns." It's a -40 moodlet. If bitten or with the Zombification potion, they'll be a zombie for a couple days.