How long should you lift the same about of weight for?

Until you can complete 12-15 reps without failure in my book.

That being said, I don't do a general 3 sets of X reps. I work out with a trainer and our goal for each exercise is 8-12 good reps and then there are normally some longer holds etc at the end. If I get to 12 without making a funny face or letting out a grunt, next time my weight will be increased.