It is time to make some changes to your workout program when you feel tortured by your workouts, and you do not want to do them. You do not want to make your fitness life like your work or your job life. This is when you will begin to miss your workouts, and start to have set backs.
When you begin to get a slow down in your progress it is usually because your body has adapted to the exercises and the workout that you have been doing. This is what your body does, it wants to adapt to the stress that you are putting it under. Find the motivation to workout harder.
It is important to change things up every 4 to 6 weeks at some level. This way your body will have to stay on its toes. As your body adapts then it will have no choice but to develop and get stronger.
You do not have to make huge changes because it may only take some small changes to make a big difference. Things like changing the order of the exercises on your workouts is enough to activate a change in the way that your body responds to the workouts that you are doing. Always keep this mind and be sure that you are always making progress. So to get killer abs you need to stay motivated.
I change things up in my workouts all of the time, and it is one of the things that keeps me coming back for more. Keeping things interesting is important to my motivation.