When you configure a membership payment button you have the option to create a "Buy Now" or "Add to Cart" type button or a "Subscription" type button.
1. "Buy Now", "Add to Cart" type buttons for membership payment
If you want to charge an once off payment for a membership then "Buy Now" or "Add to Cart" type buttons are the way to go. A user pays you a fixed amount for a fixed length membership in this model. Example:
i) $47 For a 6 month membership
ii) $67 for a 1 yeah membership
When you follow this model you should set the expiry of the membership level to a fixed length (example: 6 month). You can set the expiry time of a membership level by editing it and setting a value in the "Subscription Duration" field.
After the specified time (6 month in this case) has passed by for a member his/her account will automatically expire. The member can then choose to renew the membership by paying for another term. If the member pays for another term while logged into the system then it will recognize the member and will update his/her profile (this way the member does not need to go through the full registration process).
2. "Subscription" Type buttons for membership payment
If you want to charge someone on a recurring basis to be a member then using a "subscription" type button is the way to go. In this model a user pays you an amount on a recurring (eg. per month) basis. Example:
i) $29.95 per month
ii) $99 annually
ii) $14 for two weeks then $19.90 per month
When you follow this model you do not need to set an expiry for the membership level. Edit the membership level and select the "No Expiry or Until Cancelled" option for the subscription duration.
Why? The way it works is that when members subscribe they will continue to pay for the service until they cancel the subscription. When they do cancel the subscription, PayPal will notify the plugin which will deactivate that particular member's account. So there is no need to check on a recurring basis if a member has paid or not because if they don't pay their membership will get cancelled automatically.
In this model the membership payment is paid out on a recurring basis so there is no need for membership renewal.