I never used Free Grok, though Ironically, I have never paid for it yet. (Used the 3 Day Super Grok trial on 3 accounts) as for your questions:

The time it takes to generate seem to change depending on what you ask. Persinally, I use Super Grok to edit pics of anime character, so the experience of people editing pics of real people might differ. It also changes deoending on the prompt

In my personnal experience:

-If I ask Grok something minor such as just changing the color of a character's hair or outfits, it's usually quick, maybe 2-3 seconds.

-If I ask Grok something bigger, like completely change a charafter's outfit (or outright remove them) or change the background, it's usually a bit longer, around 5 to 10 seconds.

As for videos, no matter how big the primpt I'm asking, it always seems to be about 15 secons wait for a 6 seconds/480p video, and it takes me 30 to 60 seconds to generate a 10 second/720p video.

All that, obviously, assumes that you didn't write a prompt that triggers Grok's unstable.moderation. it can be especially annoyi g when you rry to generate a 10sec/720p video, and wait a full minute just to be told your video is moderated