Weeel, the shiro an should keep for 3 days in the fridge, but you can freeze it if you made a lot (which I did) and defrost as needed.
The other ingredient is gyuhi, not too sure about how long that takes to go bad, it's rice flour, sugar, water, and starch. But Gyuhi doesn't take too long to prepare, no more than oh... 15 minutes from scratch maybe? Including letting it cool some, maybe including preparation time. can't quite remember.
So yeah, Ima bring the wagashi I made to work later. Remind me to upload some pics of the new wagashi, before I forget. Way simpler because I couldn't be bothered.