It all depends on what else is going on in their credit. The weaker all the other factors are, the bigger dip. If you implement credit strengthening moves before hand as well as following best practices after, you can keep your score from falling too far.
My friend who went through a foreclosure and I did a little case study on his credit during the process. Pre-foreclosure he was at about 730. It dipped to 648, probably about 9 months after he stopped paying and 1 month after he moved out(Which we figured should be the low point). When he checked again less than a year later, it was back up to the pre-foreclosure range.
