There seem to be problems with instant record (ie pressing the record button) and nobody seems to have come up with a reason for it. There is quite a bit inconclusive discussion on the subject here:
Assuming you pressed the instant record button just once, it should have recorded until the end of the programme.
However, I have been having problems with this function for several months now. May I suggest that you monitor the outcome of future “instant record” events and if you have any more (similar) problems, report them to Humax.
Setting from the guide, I successfully recorded the whole of the Olympics opening ceremony which was about 4 hours I think, so there is no 2 hour limit there.