In NY, we had the state come every 90 days to validate and test our pasteurizers. Our internal system had us open each HTST every 3-6 months (depending on each one) and we would change our work gaskets, inspect cleanness. The frequency of use is how I would determine how often to open the unit. If it’s used everyday, 24/7 - 3 month check in my eyes would be sufficient if there is no problems - no cracking of plates, gaskets, etc. if you’re having sanitation issues or quality failures, opening up the press and looking for a debris is a good tool that could tell you some info.
As for the block cracking… I’ve personally never seen one crack ever. I would maybe check with the manufacturer to make sure when you close the press, that you are not tightening it too much and are in fact closing it to spec. Sometimes people forget to measure before opening the press and when putting it back together, do it too tight. Doing this many times will lead to cracked plates and I would think eventually to a cracked block.