While I agree with TaaRt on a 10 minute minimum, I could see something smaller, such as a 1-2 minute minimum before disliking, as it wouldn’t encourage games to avoid dislikes by making players uncomfortable through flashing lights and the like.
There was another proposal on the forum (albeit 4 years ago) that proposed that players explain their dislike, which, yes, could very likely be botted, but then again, it could be a requirement that the user be registered (1 hour/day?) before being allowed to review a game.
(Review proposal is here: Reviews section)
Back on topic, a 10 minute requirement isn’t feasible for people to accurately review a game, and as has been mentioned before, bots can theoretically wait hours to dislike spam if that’s the requirement. A time limit isn’t going to affect them, and if the idea is that they get disconnected for being idle, the botter could simply add a few lines of code that tells the bots to move around so they don’t get kicked.