I googled this, and I found only that shelf lives are around 2-3 years, but the enzymes use to work even after 5 years if you store them properly. I'd like to know how long I can use an enzyme in solution? Is it hours, days, months? If it is a lot shorter than the shelf life, then why is that?
The product manufacturer can define a suitable shelf life at a stated storage condition because they've performed stability studies that confirm the product in question remains suitably potent in that timeframe. Often times an expiration date ends abruptly at 1-3 years because the manufacturer chooses not to test out any longer. So that's to say, if you receive a protein with a shelf-life of 3 years, if stored properly the protein may be suitable long after.
It's a fairly different question, however, if you took and reconstituted the reagent, because chances are that stability wasn't tested in that form. In order to make a claim about the stability of a processed/reconstituted protein you need either a statement from the manufacturer, data from someone who has tested stability on it themselves, or you need to test stability for yourself. The fairly short answer is you can't know, and the answer depends on a lot of variables such as the structure, concentration, diluent and storage temperature. All such variables can cause structural changes to the enzyme, i.e. protein degradation, that will result in a different activity for that product.