This probs won't be a good answer but I guess A) once we have a pretty good grasp of what he actually did and did not do during his leadership (often secrets may be revealed after a figure has died that informed their decisions while they were alive). B) I'm pretty sure books have already been and are currently being written about Putin, given time though they will converge into i suppose a 'canon' view. C) public appreciation is trickier, since it depends on people not getting personally offended... For example here in the UK trying to praise Margaret Thatcher in public is incredibly divisive since there are many people still alive who had to deal with the impacts of her policies. Obviously Putin is in another country so assuming that you're not living in Russia or Ukraine or whatever it'll take less time for people to treat him objectively, but I have no idea how long that would actually take

I know that really didn't answer much but even talking about figures from thirty years ago can be contentious to some while fine with others, so it's really hard to put an exact timescale on it. Maybe give it a generation? Maybe even two?