Shanti, I test +ve also, and my waters broke at 36+3, I rang the hospy and they told me to come in asap due to the +ve Group B strep.
My waters broke about 2pm (slow leak) and I was in the labour ward by 330pm. I had a couple of doses of ABs (penicillin in my hand), and as my contractions were very mild, I was induced with syntocin about 8pm, full on labour kicked in at 9pm and DS was born at 11:12pm so it was all VERY quick.
I'd only had my test a couple of days before, but I understand that seeing as the test is routinely done around 36 weeks, if you go full term or even just a few weeks later, the infection may no longer be in your system.
My understanding is that once your waters have broken bubs is at risk of the Group B strep, so likely you need to head into hospital sooner rather than later, but check with your health care professionals recommendations