How many revisions of a board design will a professional make?

Say you are designing a PCB for a laptop motherboard or some similarly complex design... How many revisions will you make before mass production?

In my company we have the option of spending man-weeks to do more simulations etc to avoid mistakes in the PCB, or just forget most of the simulations and just make some test models to try out. What's the best way to decide how to balance these approaches?