If the protease you are studying is a generic broad specificity protease, then you have lots of choices as to the assay, either one of the commercial ones or you can just do it yourself. We used to use azocasein and just measure color release, which was simple and really cheap. But there are many others. Just do a quick search on protease assays and you will see plenty. Protease assays are generally very simple.

However if your protease is site specific (like TEV or similar) then this is entirely a different matter and you have to generate an assay that focuses on cleavage at the position rather than just degrading protein in general.

Thank you for your answer.