Welcome to the forum
It isn't very likely that a mistake has been made with the diagnosis if that is what you are wondering. I don't think there's any rule about how long lupus biopsies will take to be evaluated.
It's a great pity the dermatologists didn't think to give you a little bit more information and assurance at the time but you have found your way here which is the best thing that could have happened.
The Plaquenil is all that is needed in most cases to get the skin sorted out. You should see some results within a couple of months. When is your next appointment?
They should have done a CBC more specialised blood tests ( and urine) to see if there are any other signs of disease. They should also have questioned you about your general health and past problems. That is to make sure that the lupus is limited to the skin. So have a think about your general health. Sometimes the lupus is not SLE but there can still be a few symptoms like fatigue, joint aches anemia, or even slight lung problems. The Plaquenil woud help with those sort of symptoms as it is first line treatment for SLE too.
Sometimes discoid lesions are the first sign of systemic disease so it is important to know what the signs of worsening disease might be and to have regular tests. Plaquenil has an important role to play in modifying or inhibiting disease progression and reducing flares.
You will be unlucky if the Plaquenil doesn't bring great improvement in the skin so long as you are very careful about UV exposure. Look for an acceptable broad spectrum sun screen for both UVA and UVB and use it all the time. Winter and cloud do not reduce exposure all that much.
The steroid cream should be used very sparingly and not for long periods because it can damage the skin long term.
There's another antimalarial that can be added to it if you are in the UK or USA which almost always works if the Plaquenil isn't enough.
You will be extremely unlucky if the lupus does progress to a serious systemic form but even if it does, there are several effective medicines these days to get it under control.
Remember that almost all the people who post here have problems. All those who are living normal lives with their lupus well controlled rarely post on forums.
Take the view that it's lucky you got diagnosed fairly quickly. The longer it goes without treatment the harder it is to get rid of. You might not need to be on the Plaquenil for the rest of your life although you will always have to watch out for the sun. But many people take it life long without any ill effects.
If the tests I mentioned above were not done then get them done at the earliest opportunity.
Do you remember what antibiotic you took in January? If you need antibiotics in future avoid the sulfonamide types if possible, and minocycline.
I hope this information has reassured you. If you need any additional help, just ask. Keep in touch and let us know how you get on
Good luck !
Clare