For Pay-As-You-Go  Plans:

The time is counted when your account is logged in. For example, if you purchased a 3-hour daily limit plan, your account is allowed to be logged in for 3 hours per day. You may log in for multiple times as long as your account's accumulated log in time does not exceed the time limit. Please remember to log off your Mac account by using the Apple Icon > Log off after each usage. 

Note: Even if you are disconnected from your server, your account is still logged in and your daily time limit may run out.

To avoid usage interruption, you may purchase some Pay-as-You-Go Credits and your account's daily time limit will be removed. That means you can use your account beyond your daily subscription's time limit. The additional time you used will be charged using your Pay-as-You-Go Credits.

For Pay-as-You-Go Plans: Please read the Pay-as-You-Go Plan page for the detailed service description.