How much network bandwidth your Forms application will need is dependent, in large part, on how your Forms application is written. When Forms transitioned from Client/Server (CS) to Web it changed the fundamental way developers write Forms. In a CS environment, you could make multiple trips to the database for validation or Post-Query display of information with little or no affect in the performance of your form. With a web deployed form, all those trips to the database will definately slow down your forms application. You have to remember that all communication between the Application Server and the Client occurs through standard HTTP communication methods (GET, POST, etc). Therefore, each time your Form hits the database it does so through HTTP methods with the Application Server. Additionally, each time you navigate from one item to another in a form there is network traffic between the client and the application server. All this traffic eats up bandwidth.
Another factor you have to keep in mind is the number of users you will have with your Forms application. If you have a large number of users, you'll want to allocate a little more bandwidth to your Forms application.
There is a very good book that was published shortly after Oracle introduced Web Deployed forms. The book (Oracle Developer: Advanced Forms & Reports) can still be purchased and it provides valuable insight into how to design your forms for the web.
Another good resource (and free) is the How to Tune your Oracle Forms Server Applications white paper published by Oracle.
If you have a My Oracle Support account, take a look at the following documents:
* *152508.1* - How to Run Oracle Applications Network Test
* *287237.1* - Bandwidth Per User Session for Oracle Form Base Web Deployment
Hope this helps.