That's great
Well it all depends on what the market bears.... figure out what you charge for an hour and add a bit and divy it up by 4 or 3 .... Let's say you charge $75/hour then charge $25 per 15 minute reading.... maybe you want to charge $20/reading. Maybe you only want to read 3 people an hour.... up to you. (It is tiring)
3 hours is plenty... 4 tops. Remember you will be reading 4 people an hour so that would be 16 people.
If you have to travel far you can add that to the price. I would get there early and give the hostess a free reading.... pass out cards so you can get another party referral for another day.
When I began doing parties I charged by the hour... but I had corporate deals or hostesses who were footing the bill....so when others asked I quoted them my prices but I found out it wasn't the best way to market it...
it sounds better to say $40 for 20 minutes.
Test the waters.... don't undersell yourself or your time.
Have fun!!!