Looking through my passport, at a page that the US DHS seem to like using, I see three US entry stamps:

These are:
- Arrived 4th April 2014, type WB, until 2nd July 2014 (3 months)
- Arrived 15th June 2012, type WB, until not completed
- Arrived 9th November 2012, type WB, until 6th February 2013 (3 months)
The first and the third seem fine - I was stamped in, and a date to depart by (until) was either hand-written in or stamped in. Nice and clear when I needed to be gone by.
However, for the middle one, they seem to have forgotten to write in or stamp in the "until" date.
As it happens, I didn't notice at the time, and I think I left about 2 weeks later (if memory serves). However, what if I'd wanted to stay longer?
As someone with an ESTA entering the US under the Visa Waiver Program (VWP), how long am I allowed to stay if no "until" date is given on my entry stamp?