I have very painful swollen front lower gums. i'm taking metronidazole 200mg and painkillers. how long before the swelling and pain will go down?

Depends: This will vary depending upon the extent of gingival infection, the organisms responsible, the state of your immune system and other factors. Consult with your dentist. They will have a better answer for you.

Swollen Gingiva: Dental issues cannot be resolved with antibiotics and analgesics. Firstly, the antibiotic you are taking will have little to no effect on the bacteria that are present in your mouth. You may have calculus deposits or periodontitis, conditions that need to be treated by a dentist to avoid losing your teeth. Please call a local dentist for an appointment.

Swollen gums: Resolution of inflammation and infection depends upon your general health, cause and extent of the underlying problem, what procedure was done by your dentist (assuming you are not just self medicating) and how well you keep your teeth clean and follow your dentist's instructions. As you can see, there is just no simple answer to your question. Your own dentist is the best one to ask.