Translation:They will work on the farm for a month.
I think it is the fault of the English language rather than duo. We would never say "they will work during one month" it mixes up the tenses but it is a mistake many of my non English friends make. You could say "they will work for the duration of a month"... although you'd never say it in normal conversation, because we are lazy and remove the unnecessary bit ie. "the duration of" to be left with just "for". Hope that makes sense, sorry if it's a bit garbled I've just woken up!