The letter will tell you what time to get to the exam site, typically around 7-7:15. You will want to arrive earlier than that just in case something goes wrong en route. The exam will probably start much later than that time. Lunch time is supposed to be one hour, but it can last longer because the proctors don't always get back on time.
You won't leave before 4:30 unless you really know your stuff ...or you give up. If you are still there 15 minutes before the end, you should expect to stay another 30-45 min. Leaving the exam site after 5:30 is not unheard of. Trying to get out of the parking lot can take a lot of time too if there are limited road exits.