Exposure time is a function of your scope of work. Historical DOM reflects the various sales in the market, the individual seller desires, and the market reaction to the individual sales. When you are estimating the MV for a property, with Exposure Time, you are forecasting that, based on a "general" analysis of market trends, the subject will have a MV of $X if exposed on the market for Y days. Y days is reflective of a realistic market trend, again based on your definition for MV as expressed in your report.
So, you have the definition of MV that you have to consider, for example, the most "probable price" someone would pay as opposed to a definition stating "the highest price" someone would pay.
On top of that, the assignment may place constraints on your Exposure Time. It is not uncommon for REO assignments to have quick-sale requirements. The same applies to relocation assignments.
All that being said, the 30-day sales in your market may be reflective of the typical arms-length sales, or the 140 day sale may be more reflective. Time to look harder at the sales histories, make phone calls, do historical CMAs, etc to get your answer for historical trends, recognizing that historical trends may not be reflective of the current trend.