Increasing weights isn't a simple matter of timing--"every week, 2 weeks." It's a matter of when it is no longer challenging you to do that last rep in the last set. If you can do all 5 sets easily increase your poundage to the next level--i.e. 5 lbs for 12 reps, 8 lbs for 10 reps, 10 lbs for 8 & back down again. If this is easy (not overly challenging) for you increase to 8 for 12 reps, 10 for 10 for & 12 for 10 reps.
Hope that helps.