Start checking it once a week. Stick your finger in good and deep or use a small stake to see how deep it's moist. You should feel that it's 'kinda moist' about an inch down when you need to water againIn a South-facing window it should need water more often than once a month. When you water make sure that a bunch of water comes out, not "not too much". A thorough soak.
Move it as close to the window as you can. They want some actual direct sun on their leaves at some time during the day, especially as far North as you are.
I think it needs more light and more water. Hold off on the fertilizer until it starts looking better or Summer, whichever comes first.
