2 hours is perfectly acceptable, just remember that you will be using this time for a quick catch up and congratulations form your guests, and then your photographer will get your session really started. Ensure you have somewhere for your guests to go once they are no longer needed for shots, such as a bar area or picnic set up, or how about giving them some ride passes in the park! Make sure you have the chat with your venue what they can offer for your guests to make this time as breezy and fun as possible, theres nothing worse than being stood around waiting for a room to be ready and nothing to do!