8 hours seems extraordinarily extreme for 172M rows. It seems like there must be more to the story.
Are you changing the clustered index on the table? Does the table have other indexes?
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.