Monitor your simulation's progress, graded against it's own "best time", with this useful utility to monitor simulation timings, both elapsed time and projections for different stages of completion.
Admittedly, the modelling software can take quite some time to complete your simulations, but you have you take into account the amount of work it is doing on your behalf. Some performance figures are reported in the Performance section.
Nonetheless, there are ways to maximize the performance speed of your simulations, such as limiting the graphical displays to data checking and relying on text output files for post-simulation analyses. Also, convert all your binary GIS input data to ascii, it's a lot quicker for the modelling software to read.
You can also limit the information display outputs and debugging file outputs when you are sure that your model is working properly. You can set all these through your Preferences, where you can also request email notification of your simulation ending, so that you don't have to wait around for it to do so.
If you do want to keep an eye on progress then this Simulation Timings window keeps you up-to-date with your simulation's performance expressed as Time per simulation day (a single iteration of your model) and compared to the fastest time for an iteration thus far, re-expressed as a percentage and displayed on the progress bar at the top of the window.
The time per iteration is then used to predict completion times for the current simulation year, 1 (whole) simulation year, 5 simulation years, 10 simulation years and completion of the whole simulation.