Hi PaulS,
There really isn't a fixed number of years that will guarantee readiness for practice ownership. The general range is 3-10 years of practice experience. Banks like to see at least 3 years of practice experience. There main variables that you personally will have to work through are:
1. Are you comfortable performing MOST surgeries and medical work-ups independently?
2. Do you have finances in order, good credit, down payment, business plan?
3. Have you found a viable practice to buy or start-up?
I personally worked as a solo vet associate for about 3.5 years and purchased an existing practice recently. I became extremely proficient at surgery and handling a busy practice and that helped me decide to take the plunge. There are A LOT more things to consider, but I hope this will give you a start in the right direction.