Well, as I suspected, Siteground confirmed that my GoGeek plan will not accommodate ES with more than 4 simultaneously logged in users (and this would be users logged in 24 hours per day). Their response:
If the numbers you've provided are correct you will indeed need a more powerful hosting solution.
On the subject of when the site would actually be throttled down, they said it would be throttled down after meeting the monthly max of 1,200,000 executions, stating that if I meet this limit 2 weeks into the plan, the site would be shut down for the remainder of the month, or until ES was disabled.
Given the stats I provided in the screen shot at the beginning of this post, I can assume at default settings, that ES will make 7 executions per minute per logged in user. Let's say the site is busiest during normal business hours, 9-5 weekdays. That's 40 hours a week. Call it 160 hours a month. If I have on average 5 users logged in simultaneously during those times, that would amount to 336,000 executions per month - within the limits. That would set the max at about 18 simultaneous logged in users, on average, during those times on the GoGeek plan. Of course, that would mean no other logins outside those hours. But these are hardly real-world numbers. Hard to say what would actually happen without looking at a real month's worth of stats. (I think my math is right, if anyone wants to check it.)
Seems if anyone wants to use ES with any expectations of growth beyond these numbers, a different host/plan would be in order.
And Mark, regarding your earlier comment: "As for your questions regarding session life time, yes when an ajax call is made to the server to check for notifications Joomla automatically assumes that they are still "alive" and won't automatically log them out." Does this ajax call occur only when the user's browser is on the ES pages, or will it occur even if they leave ES pages and go elsewhere on my site, still being logged in?