How do I know when is the right moment to stop using a specific css vendor-prefix?

I know they don't do much wrong where they are but, when should I stop using some prefixes? What are the evidences that tell you that you don't need it any more?

For example, I keep seeing many people that keep adding the -o- all the time, despite the many versions that opera has been working with -webkit-

I say this because of 2 reasons:

Prefixes make your code heavier if you are concern about download time (specially if you are working with keyframes animations or transformations).
The code is less clean and more difficult to read.

That know, some specific questions: should I keep using -o- in any case? (I don't know if it is still important for mobile opera). How many versions of unprefixed must past before I can remove, lets say, -moz- prefix?

I know answer to this will be a little personal but I wonder if there is a rule of thumb that everyone knows and I don't.

Solution

It's the developer's choice, ultimately. Keeping in mind the target audience, if you have a lot of Opera Mobile users than of course you'd want to keep around -o- prefixes.

If you use something like Autoprefixer, you generally don't have to worry as it will do that part for you. I believe the general rule of thumb for those that "support modern browsers" is going 2 versions back of each browser's latest. That knocks a huge majority of -moz and -webkit prefixes from back in the day. Anything more than that is usually for compatibility's sake, and that kind of stuff should have audience data to back it's merit up.