How often can that be used, in a practical sense? Could one give the prayer a code word, and say the code word every time they are leaving home, such as for going shopping or other errands, in addition to locations for extended periods of time like schools or workplaces?

We think this is a fine idea. The human perspective would be that this is unlikely to even be possible and would certainly be too great a burden for the divine, to be watched over every minute, in so many varied circumstances, times, and places. But you are forgetting that the divine plan is to build in the potential for such oversight and protection, in assigning you guardian angels and spirit guides, as well as having a link to your higher self within the divine realm, albeit not a robust one allowing two-way communication that you can perceive. But your higher self always knows what you are doing and why, and can do many things on your behalf. But as with all beings of light within the divine realm, including the angels and Creator, there must be an invitation. You must have a desire for us to act and then do so, as with acts of prayer. The way to think of this is, if you are planning an impromptu shopping excursion, you are not bothering God to go along with you. The reality is, your guardians and your guides will automatically go along with you, mostly to just hover and watch what happens, not being empowered to act on your behalf necessarily, unless there are prior prayers in place requesting particular things to happen or to be warded off, as the case might be. So this oversight can be made as detailed and elaborate as one wants, to get the most from your divine spirit team. And that is a birthright you enjoy as an extension of divine consciousness that happens to be residing in a physical body for the time being. You need this kind of protection much more so being in the physical than as a light being.