How do I know when it’s time to get a divorce?

At the outset, you should be absolutely sure that your marriage is beyond saving. If you are uncertain or if there is any chance you and your spouse may get back together, go see a marriage counselor – not a lawyer.
A counselor can help you and your spouse work through emotional and marital issues. That is not the role of your divorce lawyer. I’ve heard that if your marriage is genuinely ended, you’ll know. Then, and only then, contact a divorce lawyer.