I usually change mine up a bit every 3-4 weeks, and it has produced great results. I assume you are doing both weight training and cardio?

The best thing you can do is change up the basics every time you work out. For example, if you work out 5 days a week, then you should be doing a different type of exercise each of those 5 days. Also, you should alter the schedule of those exercises from week to week. (I.E. if you usually run 3 miles on Monday, move that to Wednesday. And if you usually do Leg exercises on Tuesday, move that to Thursday.) Just totally mix it up every week the order of your exercises and the types of exercise.

If you do leg exercises one week and you do:
squats, leg press, leg extensions.

Then the next week, you should do lower weight, higher rep squats or leg presses, or higher weight, lower rep depending on what you did the week before. Do some leg curls, calf press, climb and descend stairs for 20 minutes. The next week, mix it up again. The best thing you can do to stimulate fast-twitch fiber muscle growth is to change your routine, and I recommend changing as often as possible. Every time you do the same muscle group, change your exercises. Obviously then, you can revert back to ones from a previous week a couple weeks later, but you'll need to learn a lot of exercises to keep it up. I'd keep a journal though so you know what you did the last week and can easily change it up.

Also, after you exercise a muscle, wait at least 4 days before doing it again. So don't move your Friday workout to Monday the next week. Happy training to you.

You should change your regime when you get bored of what you're currently doing. Switching things up will help you stay focused and not lose sight of your goals. The amount of time will vary from person to person.