Sounds like your mom is trying to keep you away from the degeneracy that is common in college, it's all with the best intentions no doubt, but what you're asking really breaks down to the fact that it depends on the individual.
I drink, chill, read, socialize, play video games, smoke dope, write music, etc every chance I get, and I'm in graduate school and also working 30-ish hours a week. I can still pull my weight and then some. But I am also less vulnerable to stress than most, and that makes a difference.
I say just go and wing it and see what works for you, but be careful. My undergrad was drugs and partying at all times except when absolutely necessary, and I suspect that is what your mom is trying to keep you away from--which is no doubt a good call. Undergrad was the best time of my life by far, but despite the fact I graduated on time I screwed myself over pretty bad in the process and am 4 or 5 years behind where I envisioned myself being in life as a result.
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I know I made them a promise but those are just words, and words can get weird.
I think they made themselves perfectly clear.