ClassPass is a monthly recurring membership, so it will automatically renew on the same day each month. If you sign up for ClassPass on the 15th, your membership will renew on the 15th of each month after that.
There is no long-term commitment for ClassPass. You are welcome to cancel your membership or put it on hold at any time, as long as this is done at least one day before your cycle renews. You can suspend your account by going to your Account Settings.