Love_XYZ
It's weekly. We mostly get 2 prompts for each week, sometimes only 1.
We get the new prompts on monday so I'd guess you can't join the previous one after the new one gets live. Never tried it tho. The results will come a few days later. Oh and since webnovel is the english site for the chinese quidian time is in UTC+8.
As far as I know there's no quota for how many chapters you ought to write during the contest, no minimum no maximum. Of course if you want to win you need to get the attention of the readers so good quality chapters and a stable release schedule is advised. Also doing a little bit more, like 2 chap/day on the first week is usually appreciated by the readers even if you go to 1 chap!day after.
Technically you don't HAVE TO continue your novel. Of yourse if it did poorly it's understandable to stop. If you wanna do a short one with only <10 chapters you can do it. I don't think the majority of the community would like to see that in this contest so you probably wouldn't win but you can do it. And if you win, the novel will be featured on the front page so you can get a pretty sizeable reading base for it. Why would you stop if that happens?