Then you have time, which is easy: there are 3600 seconds in one hour. So to convert L/h in m$\small\mathsf{^3}$/s, you have to divide your data by 3,600,000: 1 L/h = 2.7778 x 10$\small\mathsf{^{-7}}$ m$\small\mathsf{^3}$/s.
You can ask this sort of things in WolframAlpha; the trick here is to "convert" mm/h, assumed by the engine to be a speed, in L/h, the volume flow rate hidden by the mm/h unit.
Golden Rule Nr 1: you can't manage what you don't understand
GRNr2: You can't understand it unless you measure it.
GRNr3: You can't measure it by sitting in an office.