It depends upon the bounce rate (How much time the user spends on website before closing the website or opening another). A bounce rate of 25 Sec means on average user will spend min of 25 sec on website.
So, 30,000 hits a minute with 25 sec of bounce rate mean an Average of 15,000 users online at any particular moment.
Time consumed while the website is loaded into the browser can be neglected once you consider bounce rate.