How do I say "most of the time" in Mandarin?
一般
Tuttle Learner's

1 generally speaking, ordinarily
每星期一上午我们一般都开会。
Měi Xīngqīyī shàngwǔ wǒmen yìbān dōu kāihuì.
We usually have a meeting every Monday morning.

Guifan defines this as:

3 形 占绝大多数的
学生们一般都很尊重老师。

占绝大多数的 can also include time here so there's no need to specifically emphasize 时间 unless you want/need to.
You can even opt for a 一般来说 if you're in the mood, which KEY defines as:

generally speaking, generally, by and large

and Guifan gives the following example sentence:

一般来说, 雨天他是不会来的

Which roughly translates to something like:

Usually, when it rains, he won't come.