Depending on the server reach (population worldwide), the primetime will vary.
For typical WoW server reaching 1 continent, the primetime will start from 7pm untill 12pm. Peak should be around 9pm.
If your in game involving european and american player on the same instance, primetime will be much longer due to the timezone... and there will be 2 peak during the day.

But what about the rest of the time?
What are the advantage / disadvantage?
(note: the word theory here mean stuff we often ear from people)

Theory #1: Less players soo less grouping opportunity...
While somewhat true, it's also mean less competitions to form group. Howver by experience, it's usually way longter to form group.

Theory #2:
More ressources? Well, not in all game. Some game balance the number of ressources compared to the number of online player on the server, in the specific zone or by the number of gathered node.

Theory #3:
More mob? Same than theory 2 might happen, but well, less chance to have griefer!


I'm sure we can think about many aspect and theory, some can be biased on feeling, and most of stuff related to spawnrate can be hard to evaluate.

Besside that, playing off-hour was great for me, allowing me to do stuff at my own pace like gathering and crafting.
It's also cool to grind mob when spawnrate is decent.

Anyone else can share is feeling about difference of gameplay during primetime/off-hours?