A question personal trainers get from time to time is....how hard should I be working? A simple question but one that isn't always answered correctly. Most people who seek out a personal trainer do so because they have tried exercise on there own and for one reason or another did not have success. Most of the time it's not lack of motivation, but the lack of intensity and direction during their workouts that lead to poor results.
So, the answer to the question would be....it is relative to your current fitness level. But....you should always strive to work at 70-85% of your CURRENT work capacity in order to achieve results. If you aren't working in this range you will most likely only see minimal results, leading to discouragement and possibly abandoning exercise all together. Your fitness level will increase over time so therefore your workouts should be adjusted to accommodate the increased work capacity as well....keeping you at the 70-85%.
Hiring a personal trainer can eliminate these pitfalls and keep you on a training program that continuously keeps you working at YOUR current work capacity to achieve maximum results.