For this forum to be helpful, I personally think that you may need to be more specific.
try and answering the following questions
what is the problem you are experiencing?
what kind of answer are you looking for?
from what I can imagine, by running half model, you mean making a two cavity mold a single one for saving time purposes??? this can be achieved by using occurrence numbers (see tutorials).
if you mean that you want to see results of what is going on at 25-50% of cycle time during the injection molding process, you can run the complete model and then focus on the results related to time to see what happens at 25-50%.
if you mean you don't need the rest of the model and you want to save time since you only want to see how the plastic flows at the 25-50% of the cavity, I guess you can edit your model to only include this area, however, I would strongly suggest to match whatever you have in the mold and then focus only on what you're interested in.