Different solutions:

• You interface your application with OpenERP to create invoice automatically through XML-RPC. That's the best way as everything is fully automated, but you will have to develop the interface. (That's what we do with OpenERP Online; we do one contract for every subscription and one invoice per month)
• You can use the subscription module that allow to generate documents (like invoices) every month or 3 months.
• In the trunk version of OpenERP (for v8), contracts have a new feature to allow recurring invoices to be generated automatically based on several parameters.

Not exactly sure what you are looking for, but I assume you want to charge each customer for space, number of users etc on either a monthly or quarterly basis. This is bread and butter for OpenERP. Just set up products for each service, then invoice as required. You can have pricelists, discounts etc as required.