Depending on the lighting conditions the exposure time is between 1/50000 to 1/15 seconds. However with camera apps (other than the default one) you can achieve much longer exposure times (as long as you want).
The maximum exposure time on the iPhone 6 is 1/2 second, reduced to 1/3 on the 6s. Even third-party camera apps can't take longer exposures. They can take multiple exposures and merge them, but not a single exposure longer than the above numbers.