Is it legal for my company put me on retainer indefinitely?
With that general premise, an employer is under no obligation to provide a certain amount of work. Nor do they have to keep paying you if there is no work for you to do. Of course, to every role there are exceptions. Exceptions can arise if you have a written contract that promises you will be given a certain number of hours of work per week.
Exceptions can arise if you are a member of a union, in which case your rights may vary from non-union employees. And exceptions can arise if there is something in the nature of your work that requires you to be "on call" (and therefore available and entitled to be paid) even if you are/the company is in a waiting period.
Because so much can depend on your industry, job responsibilities and union status, it's best to speak to an employment attorney in your area so that you can get advice specifically geared to your particular situation.