How much time do you give underperforming reps before you replace them?

Depending on the nature of the sales and the induction process you have in place, I would say the formula would be to train them for the first three months. This would include your process, including all for key elements, Engaging, Discovery, Gaining commitment, and Execution; product and order entry; infield practice including direct manager coaching. Once the initial three months of structured training is over, you set out clear cut and agreed to expectations. From that point I would be looking to see how well they execute over the course of one cycle, however long that may be. If they are underperforming (vis-a-vis the historical average for your previous reps), I would put them on a program and short leash with the understanding that if there is no specific improvement by the end of the second cycle, they are gone.