The point of the 48 hour wait period for phosphate removers has nothing to do with filter pressure. It's purpose is to optimize the removal of phosphates.
Upon dissolving in water, lanthanum chloride will react with the carbonate alkalinity in your water and form lanthanum carbonate which is an insoluble compound. This reaction occurs more quickly than the formation of lanthanum phosphate because the carbonate concentrations is, in some circumstances, 100X larger than the phosphate concentration. However, lanthanum carbonate is not stable in the presence of phosphates and it will convert (slowly) to lanthanum phosphate. This reaction happens slowly in your filter and takes time.
So, in order to optimize the removal of phosphates, you must add the remover and wait 48 hours. After that time, most of the inorganic phosphates will be removed and you can safely flush/clean your filter. It is always best to add PR when you have time to clean your filter properly as the built up lanthanum phosphate can irreversibly clog up cartridge and DE filters.