Solution:

1. Unfortunately, at this time we cannot set an indefinite Return Date.

2. An alternative Solution is to set a Return Date very far in the future (for example: a very far year in the future).

3. We can automate this process by creating User-defined Defaults. Enable Developer Mode and look for User-Defined Defaults, create a new record, and enter the following information (choose the date that you prefer):

Note: The field "Return (rental.wizard) exists in the Model "Configure the rental of a product"

4. Now, when you create a new Rental Order and add a Rental Product the Return Date will default to the date you put in the User-defined Defaults: