[How much time need to approve video gig?]

It totally depends on Fiverr systems you need not be worried about it. Fiverr rank your video gigs if you follow Fiverr systems according to rules.

[Buyer cancelled the order after getting the deliver!!]

if you go to Fiverr support  and submit work proof, you will  get good result

[Why I am not getting any Order from Buyer Request]

At first, you improve your gig image and then set up suitable gig pricing and description according to Fiverr update information

[How to rank my gig?]

Marketing your gigs in the social media and all time active on fiverr.

[Why does everyone work at fiverr such a low price?]

New seller wants to rank their gig and to have order quickly so they start their service with a low price.