If you want to keep the chlorine reading at the 3 to 5 level, you'll have to add it daily.
We add right after using the spa. If for some reason we go a few days without using the spa, I at least add dichlor every 2 or 3 days or so.
Using MPS as shock is fine- some people report that it helps keep their water clear. I occasionally use it, but it does tend to drop my Ph/alk readings. Using just chlorine, once I get my water adjusted, it pretty much stays at the desired Ph and alk readings.
It took a couple years, but I'm finding out that the less stuff you add to the water, without compromising your Ph, alk, etc... the better the water quality, and the less expensive.