Butter frozen at -20C will keep well for at least a year, so you would come out about even with the 250 g. Of course, a research team has done the lab work and published a paper on the subject (
http://naldc.nal.usda.gov/download/15684/PDF). They came up with a year of frozen storage with no deterioration, before presumably giving up waiting for deterioration to occur. After all, they had to have something concrete, in published form, to show that they were earning their salaries, so they couldn't let the experiment run until they began to detect deterioration.
I've never yet been misled by US Department of Agriculture research results. If you can trust them enough to repeat the experiment by dividing a 250 g package of butter into roughly quarters and freeze three-quarters of it, you might find that you're just as satisfied with the flavor and texture when you eat the last bit, just short of a year later, as you were when you ate the first serving. The butter will be perfectly wholesome, so no worries about food poisoning.
I had suggested wrapping the portions in waxed paper, but further research has shown that it isn't a good idea. Instead, try wrapping each portion in cling film before placing in a freezer bag. That will make it easy to remove only the portion for the next 12-13 weeks. Just remember to move the new portion from the freezer to the fridge at least 12 hours before use so you don't have to use scrapings off a frozen block.
I just checked the storage instructions on the box that my pound (453.59237 grams) of butter came in. It recommends freezer storage for no longer than 4 months. I've never paid any attention, though, and never noticed deterioration in quality even after longer than a year's freezing. My general procedure is to pay attention to use-by dates and storage instructions for refrigeration, but ignore both for frozen foods and go with food preservation authorities such as the USDA and any of the Cooperative Extension food labs at the land grant universities.