Will the other musicians have their gear already setup, mic'ed and ready to go?
How long does it take you to set up your gear?
Are you playing an acoustic guitar or electro/acoustic or electric guitar or both?
Will you be micing the acoustic or electric guitar amp or will you be going straight into the desk?
Do you have ideas for vibes or tones on your songs?
How many tracks of guitar are needed - for example 2 electric rhythm tracks, an acoustic track and an electric lead solo? Add on top of that a Bass track and a drum track and your vocal track with possible background vocal track. Bare in mind that there may be mistakes! Give a thought to how you want to record - will every track be recorded seperately, or will you track the bass guitar and drums at the same time, whilst recording a scratch guitar track?
Think of how you want to mix your song - you could have a basic drums, bass and vocal and acoustic in the middle, a left rhythm guitar and a right rhythm guitar and pan some lead overdubs to the left and some to the right, plus there are lots and lots of other options in between.
Im not trying to put you off as its a great experience to create - but it is worth thinking about all these things and maybe a good idea to call or meet the studio guys and ask questions to be fully prepared to make the most of your time. Time does go fast in the studio and it can add a lot of pressure to playing when you know you are running out of time. Is there a way you can purchase an extra 4 hour session and do a straight 8 hour session rather than 2 x 4 hours sessions (to save on setting up, micing etc)
Whatever happens I wish you good luck and above all... enjoy it - the studio adds a whole new dimension to your musicianship. I am a session musician so have had some great and some really bad studio experiences, so I am more than happy to help if you have any more questions on here or PM me if you prefer.