When you get the right to sue letter, you have 90 days in which to file a lawsuit. Please take note of the fact that receiving a right to sue letter does not mean that the EEOC found that your claim had merit - just about everyone who files with them gets a right to sue letter. That is their default response - it takes no position on whether you have a valid claim or not.
If you miss the 90 day window, there is nothing, and I do mean absolutely nothing, you can do to take legal action on this situation, ever. You are forever barred from taking action once that 90 day window closes. (You don't need to COMPLETE the action within 90 days but you do need to START it.) So I would recommend that you start talking to lawyers NOW. Once that letter arrives, you want to be able to take action right away if an attorney believes you have a valid claim. (Note - if the attorney is only willing to take the case if you pay him up front, that's a sign that he doesn't think your case is worth much if anything.)