“Interactions duration between Mythelphenidate and Alcohol (ethyl)?”

I've had ADHD pretty much my entire life, and so has my sister. When I came close to the legal drinking age she told me about the interaction between the two drugs and how it can cause major problems, so I am trying not to mix them in any capacity, however, student life has proven to conflict with this goal. While I won't take alcohol while the effects of methylphenidate are still noticeable to me, I want to know how long it's still possible for these two drugs to mix critically after the effects of methylphenidate have worn off. I mainly want to know this since right now I avoid any drinking if I've taken my meds that day, and often my friends want to go grab a beer after classes, I was hoping I could do that safely.