Since 16:29 UTC the panel forecasts your credits from your monitors' intervals, not only from last week's usage. Until now the first warning came when about a week of credits was left at the past seven days' pace, which for a new account with many monitors could be two weeks late. Now, when the credits would run out within about 30 days, every page shows a line such as «Your monitors use about 967,680 credits a month; the free 650,000 run out around 22 Nov. A €40 pack covers about a year at this pace, or check less often.», with Buy credits and Change intervals. The Credits page shows the same, and the owners get one email with the same numbers.
The pack it names is the cheapest way to buy a year of that usage, the same arithmetic as the cost form on the pricing section and the interval choice when you connect a hosting panel. Nothing else changes when credits do run out: an email, seven days of grace with monitoring running, a reminder two days before the end, then a pause until credits are added. Details in Credits and billing.