How many hours to have a good rank ?

Honestly, CS' ranking system is screwed because it takes a lot of time for you to rank up. You can be a player with the skill level of DMG and still be Gold 1.

To actually be better, you need to work on your skills, like grenade throwing spots or aim training.

I have 1.3k hours and still in gold nova 2. :(

It's not the amount of hours you put in it's the effort you put in, if you aim to be the best and practice and learn from mistakes, you'll get better and better.

I have 800 hours and i'am Supreme

i spend 3700 hours in cs go and i am global in mm and 8lv in faceit :D