When you respond to someone's question by saying "your guess is an good as mine" you are letting them know that you can not speak with certainty about something. If you were to answer their question, you would only be guessing. Example: "Excuse me, what time does the bus arrive?" Answer: "Your guess is as good as mine; I almost never take the bus." Neither you nor they have enough information to answer with certainty. Your next step, if it were important, would probably be to get more information so you could speak with more clarity.