If you farm well in your lane, the gold increases in a good way, when would you think of returning and buying items? until you push the first tower in the other side, or else? Thanks!

It takes experience and practice to learn all the scenarios. For mid lane, I try to back after I've forced my opponent out of the lane (or if they voluntarily leave) and I've pushed my minion wave closer to the tower. This way, my opponent loses the exp and gold from my minions to tower kills, and my own tower is less likely to take damage. I back if I can buy a core item or my health is low and I do not have anything to sustain myself in lane.

For top and bot lane, pushing your minion wave to their tower before backing is riskier because the lane is longer (more real estate between the towers). You do not want to return to your lane with your wave being overextended since this makes you an easy gank target. If I am bot lane, I try to stay in lane as long as possible unless the enemy is developing a serious item advantage over me. Coordinate with your lane partner and jungler when to go back. You do not want to back if the enemy seems ready to do dragon, for example.

In general though, never refuse to go back if you are low in health and could die any moment. Avoiding death is your number one goal.

Depends on my build and champ usually like going back to buy when:
1. I have enough gold for a major purchase
2. My tower isn't being pushed down (unless I have teleport up)
3. I am out of mana (actually, this often forces me to go back to base)

Generally, I go back when I have about 1k (boots+finish Tear;finish Philo Stone+boots+health crystal) to 2k gold (Wriggles + Doran's Blade; Doran's Blade + BF Sword; Hextech Revolver + finish Sorc boots) depending on my buy order. Though I have also farmed 3k when I'm playing against a pusher that I can push back against and my jungler never comes to cover my lane.

You want to go back when you can afford certain items, if you can help it. Having your minions pushed all the way to their tower is ideal as well. If you're sitting on 1000 gold or something with the minions pushed all the way up, you can generally safely go back.

Don't think that just because you have money you need to go back though. If your enemy in lane hasn't gone back yet, and he isn't giving you trouble, there is no reason to go back. If he leaves, and you have money/need health you should leave as well. If you leave before the enemy though, that just leads to them free farming. If you are lowish health and afraid of ganks though, go back after pushing a decent bit.

Also, as support, I would go back as soon as I have enough for a GP/10 item. Unless lane is going really well, then I will stay and harrass. But usually it is right when I have enough for my philo and a ward or 2.

This is an area I often struggle with. Either staying to long or going back and being 100G from the combo set. The best way to get around this is to figure out, based on the champ you are playing, what items you want/need. Keep track of how much gold is needed, and then try to time your back when your low health AND have the G you need, or your lane is covered and you have the G you need. If you think you need to go back, always try to get coverage, and let team know if you think your lane is put into jeopardy

I still struggle with that.

I have it figured out somewhat on my supports, that as soon as I have enough for my Philo/Heart/Lucky Pick ill base to maximize the returns I get.

On AD chars I have no sense of when, but on Morgana for instance,

I go back when I have 1700g because I can get boots and catalyst.