I make it a habit to restart my Android phones once every few days regardless. Once a day would be good if you use the phone a lot. The Valet has very low specs so it's not going to be the best at having all kinds of apps open all the time. And it's odd because I have a ZTE Fusion II (similar to the Valet, but GSM) and it just feels sluggish in comparison to my Huawei Prism II, with the SAME specs and SAME Android version. Must be the overlay ZTE uses. After the Fusion II, I'm not a big fan of ZTE.
You might also want to try pulling the battery out while the phone is on to do sort of a soft-reset. You won't lose any data.
Castrol EDGE 5W-30 is my drug of choice.
Primary: iPhone 6s 128GB, Galaxy Note 3: 5 lines, 3x $5 Android web, 1x $2.99 T-Zones
Secondary: TracFones: iPhone 4s [AT&T] & Moto E 2nd gen [Verizon]
Mobile Broadband: ZTE Z915 LTE hotspot (5GB for $10) + iPad Mini Retina w/ T-Mobile LTE