Hi,
1. order_invoice_created is the number of seconds since 01/01/1970 up to the time the order was confirmed.
{time}-X is the the number of seconds since 01/01/1970 up to now minus a number of seconds.
So you want the orders older than {time}-X , hence the order_invoice_created<{time}-X
2. The cron task you configure will trigger all the plugins and the mass action process every 15 minutes (or something else based on how you configured your cron task on your server).
Then, each plugin (and the mass action process for each mass action) will check if it needs to run or not.
So if the mass action is configured to run every minute, and that the cron task triggers every 15 minutes, the mass action will run every 15 minutes.
If the mass action is configured to run every day, and the cron task triggers every 15 minutes, the mass action will run once every day.
And if you don't set any trigger to the mass action, then it won't be triggered by the cron task at all and you'll have to click on the "process" button to trigger it manually.
Several mass actions can potentially be triggered at the same time at some point, but that's not necessarily the case. It really depends on how you configure them.