There are two kinds of subscriptions which I can think of off the top of my head. Monthly subscriptions and hourly subscriptions. The difference is as follows. Monthly subscriptions last for a duration of months after their activation and constantly count down regardless of whether or not the player is online. Hourly subscriptions pay directly for the hours a player can be online, which can be used whenever the player wishes, meaning that you are directly paying for the time you are online.
The inclusion of both would be very beneficial for creating freedom in choice for purchasing time to play the game. The major difference is that monthly is often cheaper, but doesn't account for players who rarely get on or who get on extremely often, resulting in a less predictable load on the servers. With the hourly system, players directly pay for the time they will be using up server resources, which is more precise and can be saved for when they want to play.
As someone who often plays something a lot but in short bursts, I would prefer the hourly subscription.