Answer:
when that accuracy is sufficient
Explanation:
Powers of ten are used for estimation purposes when that level of accuracy is sufficient for the purpose.
For example, we estimate the number of stars in the Milky Way galaxy to be about 100 billion (10^11). We have no idea (and don't really care) whether it is 100 billion or 200 billion or 85 billion. We think it likely that it is more than 10 billion and less than 1 trillion, so 100 billion is "close enough". That stars are being continually created and destroyed, that there are various degrees of "star-ness", and that what you see depends on where you are also comes into consideration.