How long time does it take to TRUNCATE 50 million rows?
In normal circumstances this should take very little time since it is a metadata change. Whereas a regular delete will delete all data row by row, a truncate operation will deallocate all data pages instead.

Be aware though that a truncate table requires a Sch-M lock, so on a busy environment it could take a while before it can get hold of its lock and truncate the data. If your statement keeps running for a long time, it's probably locked by (an)other transaction(s).

If you want to be sure, try it on a copy.