Unfortunately, this is the most difficult question to answer. Only you know if you are ready. Suspension readiness is much more of mental state than physical. As with any form of body modification, one must make the decision based on personal experience and research. At the same time, no amount of research or even watching suspensions in person can really prepare you for the feeling of suspension (but it sure helps).Take a long hard look at the experiences of others -- especially the "failures" and negative experiences. Ask yourself why it is worth it to you to face these outcomes and whether or not simply trying would satisfy you. If you can seriously say that it is worth it to simply try then wait and think about it more. A good test of whether anything in life is something you really want and are ready for is if you can stand to wait to have it. Infatuation fades but a serious commitment will withstand the wait and only strengthen your resolve.
Ultimately, "you'll know when it's the right time," but don't be surprised if you still find yourself overwhelmed with fear. This fear will disappear if you accept it and let it pass through you -- resist it and your suspension will not be a pleasant one as you will not be open to the experience beyond your resistance.