What is the right time to gank?

Assuming you are the mid lane, you should gank top and bottom lanes more frequently than if you had a jungler, the following are just general guidelines to bare in mind before/during/after ganking.

Before

• Make sure mid lane is not pushed and that either enemy mid lane pilled back or you killed him.
• Check both side lanes for enemies overextending and being below half health.
• Either ping and type in chat your intention to gank, if possible ask whether the side bushes are warded, this will save you time and lost exp/gold from minion kills.
• Depending on the champion you are playing you will most likely need to be at least 6 to make use of your ultimate.
• Make sure you have your summoner spells not in cooldown, successful ganks can be determined on this, wise use of ignite/exhaust and ghost/flash to catch up will almost guarantee a kill/assist.

To answer one of your questions determining when is early or late, usually mid towers are taken down in 15-20 minutes of gameplay, you should not gank if you are going to loose mid tower as it is the most important tower you have, therefore taking into account the things I mentioned above too early would be not having more than one of the aforementioned; as simple as it is, too late is whenever the side lane towers were taken down and you had the opportunity to gank.

During

• Make sure you and your teammates focus the weakest/most important target.
• Do not tower dive if the gank is most likely to fail, it is not worth to get 1x2.
• Do not ignore minions during the gank, specially if you are below 1/4 health! Beware of the purple caster minion (Specially during early game)!

After

• After a successful gank from mid, you can either push the lane you just ganked in order to get the tower gold and experience, this should be done if you can be aware that a counter gank is not going to happen and also if mid lane is not being pushed.
• If you are not going to push the tower, you should leave the lane for your teammates to farm gold.
• If you ganked bottom lane and if you and your teammates have the appropriate map control you can all get dragon, it gives global gold as taking a tower does, but it is worth more during early/mid game.

At what time do i expect a gank?

Expect a Gank anytime when you see the lane not being pushed and 1 or more of the enemy champions missing in action, expect a Gank whenever the enemy lane becomes over confident being below half health, meaning you are being lured into a trap. Expect a gank when killing enemy Blue/Red and whenever killing Baron and Dragon.
Best Counter gank: Proper warding of the map

Early game: The best shot would be at the beginning to get the First Blood. Then begin laning. It will be difficult to gank before being level 6 and you really can't afford the lost of XP. When you're mid-lane and the other midder does not takes you to much focus, you may take advantage of Ulti's like Vlad's to finish enemies on some lane with friend and go back mid. Or if you have some long range skill (Ashe arrow for example) try to signal it to friend and wish you luck.

Mid-game When reaching level 10 or more, team usually gather. At this point you can take advantage of clairvoyance or wards to detect soloers and try to get them. But usually this is the worst time to gank in my opinion since team are trying to find they mark to play together. So people solo are usually backed up fast and from many possible direction. If well coordinated with your team and fast killing, it can be a great advantage.

Late-game if you know your team and they are reacting fast: try to gank every isolated player. This is more a team fight period but sometimes you have some people trying to be heroes. Eventually if you have a fast killer like LeBlanc, you can try to do "ganks" when one enemy is getting ahead his group a little. If you can "one shot" him and teleport back without a scracth, take the shot.

Expect a gank

• Every-time you are one tower ahead your last alive tower
• Every-time you are pushing a tower alone
• Every-time you are in the ennemy forest
• When in late game opponents is fleeing in front of you (even if he has low life). Most of time they are trying to lure you to regain advantage or breaking a defensive state.
• Every-time you think you are a hero.

A couple of "dirty tricks" for ganking when you're mid:

1) After you have blue-pilled back to base and your opponent in lane saw you do that, do not come back to lane immediately: tell your jungle to keep the lane for a bit and go straight to a side lane to try and gank. This way - especially in low elos - the opponent solo mid will forget to mention that you are missing.

2) After your mid-lane-opponent blue-pilled back, push the lane and try to gank. When they are buying, most people usually forget to mention the misses.

3) If your top/bot tower is about to go down in the next couple creep pushes, it's usually a good time to gank. The reason is that people go blind when they are about to kill a tower and don't even read chat for misses (yep, been there done that myself too).

4) Sometimes, a psychology-based trick that I do is the following: when my jungle is ganking top, I go gank bot. People usually feel safe when they see a gank happening in the opposite side of the map and do not expect to get ganked at the same time.

These are just some dirty tricks that got me unstuck from the low-elo hell. Everything that people wrote before me is good too.