555 Timer Min Max Expected Output Voltage Value
The question you ask is not easily answered. The problem is that the chip really doesn't have any specs for your condition. If you look at http://www.ti.com/lit/ds/symlink/ne555.pdf, section 7.4, you can find the high output voltage numbers, but these are only given for fairly large (100 mA) currents. There is simply no guidance in the data sheet for performance with very low currents.

With that said, you'll notice that (for instance) at a 15 volt supply the high level is guaranteed to be 13 volts, or 2 volts less than the supply. For 5 volts, it is once more spec'ed for about 2 volts under the supply.

So, anything above 7 volts is about as good as you should hope for, and 7.7 volts seems a pretty good number.