Rather than wait until your project schedule dictates it's time for Computational Fluid Dynamics (CFD) analysis, allocate time now to 'play' with CFD. By 'play' I mean construct simple geometry and keep your cases relatively small, so you can get familiar with the basic features of your particular CFD software in a timely manner.

Caedium at PlayMimicking the classic pipes screensaver

The worst possible moment to learn anything, including CFD, is under an aggressive project schedule. Even worse is if your first CFD project requires you to perform a complete analysis of a complex production-ready CAD model.

After playing with CFD and working through some tutorials you will be in a much better place to judge what to simulate and make more accurate estimates for turnaround times. Remember 'playing' means exercising some CFD features because you can and not because you have to. Through this approach you'll find some CFD software features will be better matched to your needs than others, so be sure to take some notes as to which ones you prefer.

It's good to play.