According to the objectives of your study, you may decide which effect to measure (cumulative, single, or both). However, it is advisable that you read a number of papers related to your topic and the respective effect. Hereunder is a sample of such relevant paper; although old, I believe it would help you out.

Using financial data to measure effects of regulation

GW Schwert - The Journal of Law and Economics, 1981 - journals.uchicago.edu

Since the market cap is affected by many variables, significant control variables should be used. Then most relevant interaction with the dummy would give you a meaningful result.

I suggest measuring the interaction and the cumulative effect because it will reflect a clearer picture.