The best time to start a solo practice is when you can no longer take working in a law firm, or when you need to start your own practice and have the energy to get one off the ground. If you know that you have no interest in working in a law firm and you are passionate about having your own practice, then it is a good idea to start immediately.
Sometimes it is a better idea to wait. The mistake that many attorneys make when starting their own law firms is that they do not get enough experience before doing so. Working in a large law firm is a very good thing because the law firm will train you and make you a better attorney. The longer you are in a law firm (within reason), the more trained you will be and the more things you will understand. If your goal is to be good at what you are doing, then the more training you get, the better. You want to know how to do excellent work before you go on your own.