Dear Olivia Ho

Its depend on the company instructions , As Sigma Aldrich  Catalog Number CS0790 Storage Temperature 2–8 °C

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma/Bulletin/cs0790bul.pdf

ABTS Substrate Solution – Add one ABTS tablet (Catalog Number A9941) and one Phosphate-Citrate Buffer tablet (Catalog Number P4809) to 100 ml of ultrapure water and mix until totally dissolved. This solution can be stored at 4 °C for no more then two weeks or at –20 °C for at least one month.

But  the guide of abcam    http://www.abcam.com/abts-ab142041.html

Peroxidase substrate   Store at +4°C  , Wherever possible, you should prepare and use solutions on the same day. However, if you need to make up stock solutions in advance, we recommend that you store the solution as aliquots in tightly sealed vials at -20°C. Generally, these will be useable for up to one month. Before use, and prior to opening the vial we recommend that you allow your product to equilibrate to room temperature for at least 1 hour.

Pls. find the attached files

Hoping this will be helpful

Regards

Prof. Houda Kawas

Also

Houda

Make your solution, store it in  the fridge  and record UV-vis spectra each day. You will get the answer to your question. 

Once after stock preparation we can use it for 1week