。
She's been off the whole time since last Wednesday.
Literal
She [topic-は] this-time-before [genitive-の] Wednesday [from-から] continuously is-resting.
The combination ずっと~ている ('continuously [verb]ing') is the standard way to express 'has been [verb]ing all along.' ずっと ('continuously, the whole time, by far') intensifies the duration; pairing it with ている locks in the unbroken stretch from a starting point. The starting point is marked with から ('from, since') on この前の水曜日 ('last Wednesday'). 休む covers a wide range of 'resting' — taking time off work, missing school, taking a break, sleeping. Without further context, the natural read is that she's been absent from work or school — the kind of thing a coworker or classmate would mention.