If you have new site then don't do fast and if you have old website then you can do as fast as possible but make sure if right now you are getting 20 links per day and after one or tow month later you are decreasing number of links then it can also effect on your ranking so bear in mind that you have to continue your work as you are doing. And most important thing is get quality back links as you know there are big different between the quality and quantity so go with quality ignore quality.
Consistency is the key, it's okay if you get 500 backlinks in one month as long as every month you are getting around the same.
As long as you keep them relevant there really is no limit. I've picked up 18,000 over 48 hours with a Digg front page, so I wouldn't worry about it.