How long is it safe to store rice horchata for?
At least one recipe specifies “horchata will keep in the fridge for up to 7 days,” another “up to four days,” others more vaguely “a few days” or “several days.” Note that it also depends on whether the recipe you're using adds milk or not, if it “is made without milk it will keep in the frig for various days.” Probably the best piece of advice is this one:

Horchata will keep in the refrigerator for 3 to 5 days; use common sense.
From: http://www.loveveggiesandyoga.com/2012/03/homemade-horchata.html