How long will a truncate table take on 252M Rows (170GB data)?
A millisecond or so.

Truncate is O(1) - a pure metadata operation.

This is assuming there is no concurrent activity on the table.

Truncate will potentially need to wait until existing reads are done to acquire a Sch-M (schema modification) lock.

For large tables, the actual deallocation is done asynchronously on a background thread (SQL Server 2000 SP3 onward).

For some other myths surrounding truncate, see A SQL Server DBA myth a day: (19/30) TRUNCATE TABLE is non-logged by Paul S. Randal.