From what I've heard, general "accepted" policy is that there should be 12 months in between. My SIL had a C/S Sept 04, one Feb 07, and another planned for late June 08. Her OBGYN practically accosted her when she got pregnant again, and read her the riot act. She has been told that due to risk of rupture, she is to IMMEDIATELY head to the hospital at the first sign of labor, and will go directly into her section. They absolutely will not let her body labor at all. She is high risk to begin with, but this gives you a bit of an idea of how hospital staff may take it.
That said, I've seen multiple women have VBAC's after an emergency section, anywhere from 11-15 months +, with no complications whatsoever. The 11month difference was a woman who had an extrememly traumatic (and against her wishes) emergency section, and then delivered her next unassisted at home.
She has very good resources
here
In an otherwise healthy mom, taking into account, like PP's said- her general fitness, diet, pre-existing conditions, I would personally be more worried about the risk of a third section.