Je ne sais pas pendant combien de temps.

[details=“English Translation”]I don’t know how long for.[/details] better placement of the preposition for would be “I don’t know for how long” placing it at the end is confusing (another verb complement is expected)


Ending sentences with prepositions is not uncommon in modern English. In fact, one of our most famous former Prime Minister’s was prone to it:

After receiving a Minute issued by a priggish civil servant, objecting to the ending of a sentence with a preposition and the use of a dangling participle in official documents, Churchill red pencilled in the margin: “This is the sort of pedantry up with which I will not put.”Winston Churchill (allegedly)