How does flyback diode decrease the response time of a solenoid valve
You must model the solenoid valve as a resistor (33\$\Omega\$ in your case) in series with an inductor. If you like you can include a bit of parallel capacitance, but the resistance is required. 

Otherwise the simulated current after a long time will be limited only by the MOSFET (maybe to something like 30A) and the voltage will approach zero. 

For the operation time of the valve, you can consult the data sheet. Normally they will specify it without a diode, so whatever time you get with the zener will be a bit longer. You can guess that the difference in the current through the coil will give you an idea of the operation time, but there is no guarantee that even if the coil current drops to 10% or 5% of normal that the mechanical parts will have moved in that time. Some valves are fairly complex ('pilot valves', for example that operate indirectly), and the inductance of electromechanical systems often varies significantly during operation. 

Edit: Below is a PSPICE simulation I did with similar parameters to yours (I used a 1N4740 Zener and a PHD23NQ10T,118 MOSFET to save me time). Green trace is the gate drive signal (before the resistor), red is the MOSFET drain.  As you can see the MOSFET drain rises to a peak voltage of around 24V. 



In the below close-up, you can see where the bulk of the energy is dissipated and the ringing afterward. The violet trace is the coil current (scaled so you can see it on the same graph).