First off, this is a business decision, and no one other then yourself will know all the ins and outs of your business life.
The question to ask is do you think you can continue to work at the Equity level? Do you think you are ready for the responsibilites of being an Equity SM? Are you willing to leave behind some of the perks of working in non-AEA theatre?
For me the right time was during grad school. I knew that I was going to do this professionally, I knew that this is what I was good at, and I had the opportunity to work on some top tier Regional shows as a non-equity assistant, and really got to understand the ins and outs of being an Equity Stage Manager. I also spent several months observing other Equity stage managers from Broadway to Regional to Touring shows to make sure I knew what I was getting into. I did not taking getting my card lightly.
For me, once I got my card, I have been able to work constantly. But on the flip side, I am willing to move at the drop of a hat, or travel, and do really consider myself living at any giving place. If you make the decision to make a living at stage management and only stage management, there are somethings you have to forgo (this is true for any committment to the arts.) If you want to work at a certain level, or remain in your hometown, then you need to look honestly what that level pays and how often you will work, and adjust your lifestyle accordingly. If yolu have a certain lifestyle you wish to support, then you might have to consider the possibility of moving around, or working a lot more then you feel comfortable.
Is there ever a perfect time to take a contract? No. You have to just weigh everything and take the leap when if feels right.