That estimation seems correct to me.
The estimation is based on the number of emails that the campaign has sent in the last hour, so it can vary a lot, from refresh to refresh.
what happened if 60 customer send email concurrent?
Let's think a bit about this.
If you have 60 customers, that send 60 campaigns at same time and your backend settings for campaigns at once is 10 and subscribers at once is 500.
Given that the cron job that picks campaigns runs each minute, it means that in the first minute, it will pick 10 campaigns from those 60 and will start process them. If your server is powerful enough, it can finish those 10 campaigns in that minute and in the second minute will pick the next 10 and so on, so in theory, you will need 6 minutes to finish an entire round of sending for 60 campaigns with the above settings.
However, if you aren't able to send all those 10 campaigns in a minute, then things get tricky. Say you can only send 300 subscribers at once in a minute, this means that in first minute, ten campaigns are selected, and the first one starts processing and it doesn't finish in that minute. Now the cron jobs at minute 2 will run and will pick another 10 campaigns, but these 10 campaigns are actually 9 campaigns from the previous cron, and a new one, and will start process the first campaign, which in the previous cron job, was the second one.
Now the first cron job finish processing first campaign and it moves to process the second one but because the second one is processed by the second cron job, then it skips it and picks the third campaign and starts processing it.
As you see at this time, we have 2 cron jobs that run pretty much in parallel and pick campaigns from a queue of 10 and sends whatever campaign is coming next and isn't processing yet.
But we can as well end up with 10 parallel cron jobs in this way, that will process 10 campaigns in parallel.
I don't know how much sens does this makes for you, but it's crucial to understand how sending is done so that you better understand how your settings affect the delivery process.
If you have further questions on this, please go ahead and ask.