A few answers to questions:
Is Ventrilo Necessary?
Kind of. It takes me as the DM a lot less time to speak than it does to type, and helps me separate between what is "game stuff" and what is more casual conversation. That said,
you don't have to speak on it. If I didn't use it, we'd make much worse use of our time at the table.
Ventrilo (like MapTool) is free, cross-platform, and, for our little group, I can be the host for it. It's such a time-saver, and we're on a pretty tight schedule, so unless someone has another free, cross-platform voice chat service in mind, it's Vent.
What Time/How Long?
This may change as the game goes on, but right now the schedule is 11 am to 3 pm Eastern time, on Sundays during July (the 11th, 18th, and 25th). In about mid-July, I'll start talking about when we can schedule August games.
The bottom line is basically three sessions per month. Each month, you guys should gain a level.