When the interviewer is asking this question they are typically looking for an answer evolved around metrics. I would focus on two metrics to keep it short and ask what tools they have in place to assist with measuring success.

You want to avoid coming off as arrogant or that all of your strategies have been successful. This is simply not true, even if all have had a positive ROI there is an area that you could learn and improve upon.

"I like to keep it simple when evaluating if my strategy was successful or not. I first start off by viewing what the client thinks is successful in their eyes because this is the most important part for me. If it's that they wanted to increase more customers who are in the age range of 30-40 then that's an area I would use to evaluate my success. I like to look at how the team worked together to learn new methods and continue to deliver the best service while providing the best experience which leads to client retention."