HI Juan, I personally do not own a business , however, I do have two friends who have opened their own body/repair shops. First, when you say how long should you wait to open a business , the answer really depends on whether you will be doing the work, or hiring others, or both. If you are only going to own and manage the shop, then you could pretty much open it once you have the capitol and a solid business plan, etc. If you want to be the primary technician working in and running the shop, then obviously you would want to make sure you have enough years of experience doing the work and are familiar with all of the local city, state, and federal laws surrounding this type of business. The fact that you are even considering it is step one! :-)
As stated, I am not a business owner, however, would like to give you some advise to just make sure you do your research! You need to choose a location that will be successful and has a demand for this type of business .. you need to determine how much money you will need to lease a building, buy tools and equipment, and if hiring employees, will need to consider wages, and all other expenses (electricity, internet, phones, etc).
Any successful business starts with drive and determination and a solid plan of what is needed and how you are going to get there.
I wish you the best of luck!