You seem to be asking for a specific answer to a very general question. You'll probably need to give more detail of the slab thickness, time from pouring to form removal, etc. to get meaningful answers.
Rod Smith, P.E., The artist formerly known as HotRod10
The shoring jack mfg. should have specifications on their product. For preliminary design or spacing requirements, then ofcourse you would consider other issues such as soil bearing capacity or condition of supporting soils, any other specific site load requirements.
There could have a rule of thumb by shoring contractors I'm not aware of, but as an engineer, you need to ensure the re-shore shall be setup in a manner that will prevent the slab to experience internal stresses due to construction activities prior to attain the specified compressive strength. Be conservative in estimating and designing such system.
