How long can one stay on depoprovera before it cab start to thin your bones I've been on it about 11yrs?

Thinning bones. There is no definite answer to how long it is safe to take depoprovera without affecting the bones. If it is working well for you, it makes sense to continue it. If you are concerned about your bones, the simple way to assess bone health is to have a bone density test (dexa, etc.). If there is evidence of significant thinning, you may need to change the depoprovera.