I am using "Eucerin Daily Protection, face lotion SPF 30" with zinc.  The label says it is effective for 24 hours of moisturization.  But does not say how long the SPF is good for. I realize that zinc is a physical block and hence has different properties than other sun blocks, but not sure how different the protection span is.

If I put this on in the morning and, go into my office job (sitting at a desk), will it still be an effective sun block 4 or 5 hours later when I go out for a lunchtime walk?  

*Assuming: I don't wash it off, nor work up a sweat which might cause it to come off.*