Anna, my rule of thumb is that if the individual ingredients can be kept in the panty indefinitely
(vinegar, oil, salt, pepper, dried herbs, mustard)
then surely the dressing can be kept in the fridge indefinitely.
The exception here would be fresh herbs and cream, which would probably have their lives prolonged by the acidity, but for safety's sake, should be used within a few days, the same rule as their fresh state