When does minecraft 1.8 come out for PC?

Soon.

Jeb recently mentioned that 1.8 will most likely not be released on September 8th, to allow time for more thorough testing and quality assurance. Still, there seems to be a careful optimism regarding the release date - while it may not be on the 8th, it will still likely be in the near future. Ps jeb is one of the developers

The only person who knows is Notch, and he hasn't said anything at this time.

The others are correct

According to http://minecraft.gamepedia.com/...on_history most of the recent (since 1.3) major versions are released with 1 month difference to the last version (apart from version 1.6 to 1.7, which 1.7 was made in the month after the last version of 1.6).

I presume it will be released in October unless the pattern has been broken on the last major update, then they will probably release it on September