Hitting gold on a gym

Defending a gym earns 1 badge XP per minute and you need 30,000 badge XP for gold. 30,000 minutes = 500 hours = 20.833 days.

You get more items from spinning a gym with a higher badge. For rural players, this can make a huge difference, especially when combined with daily streak bonuses.

It's not only days defending that lead to gold, but also berries fed and raiding that gym. You get some XP when you get the gold medal, and item drop is 5 or 6 from then on.

The only real benefits you get is that you get a few more items when spinning it. If it's a gym that can get EX raids, then I think you have a slightly better chance at getting an invite.

I know some people try to get gold in every gym in their area. It's always good to set goals for yourself in the game.

As for how long it takes, that's hard to say.
I think you get like 1 xp per minute that you spend in the gym.
But you also get 100 xp to place a Pokemon in the gym and also earn xp from knocking out Pokemon in the gym.
Feeding berries to the pokemon in the gym gets to 10 xp per berry.

If you can do raids there, then that helps alot since you get 1000 xp for every raid.

In the end you need to reach 30.000 xp to get gold.

If you just put a Pokemon in a gym and leave it there it would take you about 20 days to reach gold. But you rarely get to stay there for that long, so you might have to go back and knock the new team out, which will get you xp. Feed berries and get more xp. Do some raids if you can and the race for gold will shorten.

Good luck.