How to find the time constant of a water tank control system?
If the "current or fluid flow" resistance is equal for both input and output then the currents are equal and the storage remains constant. It never fills. right? 



It's a like a battery charger , battery and load. The battery charge does not change if in = out. Then the battery or tank capacity is irrelevant.







simulate this circuit &ndash; Schematic created using CircuitLab

Summary

Your time  is empty volume/ max fill rate and to fill up only occurs when the outlet is reduced or closed.  If  volume fills per unit height based on maximum height velocity=volume flow/Area= 1.5 m^3/min / 2m^2 = 3/4 m/min then depends on volume to fill and flow rate current difference.   Nothing fancy.