You can certainly mod the game to increase the time limits if you so wish.
Simply change the values of "TIME" under each "MAPSIZE" definition in the distribution.ini file to be the desired number of years.
I would maintain that part of the challenge (and hence enjoyment) of the game comes from determining how to best travel so as to do as much as possible and still return within the specified timespan.
You might get short-term satisfaction from a limitless (or effectively limitless) game, but the long-term appeal of playing will most likely be significantly lessened by such a change, IMHO.
So... make such changes at your own risk
