This depends on a few things. If you do the absolute bare minimum and defeat two Unusual Hilichurls a day, it'll take you 25 days to defeat 50 of them. If you don't want to play co-op, you can defeat a maximum of four a day in your own world, so it'll take you 13 days to defeat 50 Unusual Hilichurls. If you want to make sure you always get rewards for defeating the Unusual Hilichurl, you'll need to defeat ten a day, so it'll take you five days to reach 50. You could theoretically defeat 50 Unusual Hilichurls in one day by joining at most 24 co-op games as well as farming the Unusual Hilichurl in your own world, but for the sake of your sanity, I would not recommend this.