To understand that it is not permanent. I doubt that it is continuously like that for many months. There would be times when it faded away. Also noting that state of mind as sadness can give some distance in-between it (I am talking about practitioners here - you might have to approach it in a different way if a person doesn't have a practice, but I am talking about practitioners.) A depressed state of mind - first of all to note it as a state of mind, and although we may think it is permanent, it is not always present. When we are able to get a bit of distance between a depressed state of mind and not take it as oneself, we may see the hindrance and then we have more opportunity to know what to do and what reflection helps to balance it.
Depressed states of mind can come about due to grief from loss, in which case using the D/D or universalizing technique may be very helpful. A depressed state of mind may come from self-pity, so universalizing can be very helpful. Sometimes it comes from not accepting that Dukkha exists. We have a lot of expectations of the future and life is not living up to our expectations, so if it comes from desire, we may have to reflect more about how fortunate we are and see what we already have. This helps us not to take what we have in our life for granted. That helps with desire, also reflecting on death helps to let go of our expectations for the future.
So noting the state of mind is very helpful, just as it is, and seeing that it is impermanent. Other times, perhaps a person blamed us unjustly, then we may have to do a reflection on the eight worldly dhammas and bring up more equanimity to know that we are the owner of our own Kamma and they are the owner of theirs. Then we are less likely to get depressed as a result of someone's blame. So it really depends on the hindrance that is creating it, but before we can actually see that we have to first note the state of mind in a broad way, so that we are not feeding it and we understand that it is impermanent, that it is not forever always present, as we think it is.