There are two possibilities here. One is that your work area is highlighting less than the entire sequence. There is a selectable area (shown by two notches on the timeline) that will normally be the limit of the area that Premiere exports if you leave the defaults. You can either adjust the selected work area or choose to Entire Sequence as the range to export.
The other, though much less likely possibility, is that you are using a third party H.264 encoder that is in a demo mode and limiting output to two minutes. It is reasonably common for trial modes of codecs to include a length limitation so that the full quality can be seen but it isn't useful for any major projects, but unless you've installed a third party codec, this isn't a possible cause. If you had a codec installed in the past however, Premiere may have picked it up and started trying to use it.