The rule of thumb is that if they are frozen individually, then 30 minutes before use will be enough time to thaw them out. All you will need to do is place them in a bowl of cold water and you're sorted. If they are frozen as a whole, you might want to leave them to defrost in the fridge for 1-2 hours. If you have any leftover bits of avocado flesh, you can also store them together in one bag/container/wrapped up in cling film!