Good players in higher elo start farming wraiths and wolves quite out from midlane quite early. Actually I can't explain myself how they actually manage to do so. When I start to farm Wraiths pre-Level 8 and I don't have blue, clearing the camp in a decent time drains half my mana, because I need all spells to come back to lane in time and I don't take to much damage. So my question is, when should I start farming wraiths (when I have a Mana Champ), so that it doesn't have to many impact on my lane itself? (Assuming I don't have blue and the lane is going "normal")
Your effectiveness at clearing jungle creeps depends primarily on two things: your champ, and whether you have blue.
Some champs can farm up wraiths and wolves more effectively than others. Post-6 Anivia with blue, for example, clears camps quickly and easily, whereas Kassadin has a bit more trouble. If you don't have blue, the number of champs that can easily and quickly farm camps decreases.
Above all, though, make sure you communicate with your jungler. Some junglers will be more than happy to let you have wraiths, but some junglers might want/need the farm. It's always polite to ask. Pissing off your jungler is never a good idea.
As to when you should farm wraiths, that depends entirely on the dynamic of the lane. If the lane is not pushed to the enemy tower, you should stay mid and farm; you'll get more experience and gold that way (unless you're someone like Anivia and have blue, in which case you can probably sneak away to grab jungle creeps and come out on top for it). If the lane is pushed to the enemy tower, you have four options:
1) Attack turret. Taking mid turret early gives your jungler a lot of breathing room, and gives the enemy jungler a headache.
2) Ganking top or mid. You'll see this a lot in high-level play. Most mids are powerful gankers, and this is a great way to help out your teammates (while often giving you more gold and xp!)
3) Going back and buying. Sitting on 2000 gold? It won't do you a lick of good if you don't spend it.
4) Taking jungle creeps.
Just because you can farm jungle creeps doesn't mean you should. Often a gank top or bot is more effective.
Not every mid champ can farm wraith effectively without blue, you need to following quality:
These are a partial list of champs I think can farm wraith effectively after level 2: