D.K.
I would let her sleep when she is tired and be awake when she is awake. When DS was that age, we put him in his crib after (what we hoped was) his last feeding - generally around 11 pm. Most babies have an approximately 3 hour schedules of wake, cry, eat, play, sleep. If you put her down at 7, I would expect her to be back up at 9 or so and likely need to be fed again. If you feed her later, she may sleep through until 5-8 am (or it may take her a little longer, 18 weeks is certainly within the ballpark for normal)