Just out of curiosity how long did it take any of you? I'm still practicing myself granted its easier on some chars than others.

What is there to master? You hit them when they have low HP. It's not as if you can be denied. I don't quite understand your post.

He means a high creeper score I think. How much practice is needed before you can always pretty much get a good one.

it takes a bit and you have to know exactly what dmg kills with what hero so it changes for each person you play

I'm certainly still working on it, even for my favored characters. Knowing how my damage scales is tricky, too. It's toughest at the first levels; as you get stronger, it's easier to estimate when one hit will kill a creep. And it's easier to do/practice with ranged champions, too, though that probably goes without saying.

You never truely "master" last hitting. Even the pros don't get exceptional creep scores every game. But id say 10 creeps every 1 minute (100 creeps by 11ish minutes) is pretty darn good. At that point you can say you are "masterful" when it comes to last hitting

Last hitting is pretty variable from game to game. Alot of it depends on your lane (i.e. partner and who you lane against). For example, if you are AD bot, your last hitting is highly dependent on harassing to control the lane as well as winning trades. Same goes for mid and top, but it more reliant on the jungler.

use flat AD runes. = easy last hits
and it still scale well late game with Last Whisper.

About one minion wave.

As said, it is easiest to last hit with ranged AD characters or casters that can afford to spam their spells. The most CS you can get from only laning is 107 by 10 minutes. Between 60-80 at 10 minutes is usually acceptable. It depends on your laning enemy because keeping on par or beating them is the highest priority, then you can focus on just how well you actually as last hitting.

http://www.lolpro.com/guides/game-pl...e-last-hitting
This should help you in your last hitting and the chart is accurate and helpful.