"Her father drinks tea from 6:30 to 9."
What's the rule for using 的？Is it okay to drop 的 in this sentence? Sometimes I've seen it left out when talking about people (e.g. "my dad" or "my family"), but im not sure
Leave it out for family. There are a few other cases in which it's fine to leave it out. However, it's never wrong to include it. Often when there are multiple 的, they are left out except the last one. F.e. My friend's dog is happy = 我朋友的狗很高兴。If we can infer possession from the context or the sentence, Chinese people tend to leave it out.
In the university classes I've taken, and while living in China, I've been told to omit the 的 when talking about family. Its actually a really bad word when referring to someone else's mother, using the de.