I would caution against the "topoffs" with the bottle unless your doctor is concerned about weight gain and suggests it, or if it's a once a day before bed kind of thing. Babies get to where they prefer the faster flow of the bottle and it is an uphill battle from there, they will hold out for the bottle, it's so much easier.
Around this time, they can start to get very efficient at feeds. 15 minutes every 3 hours can be perfectly normal for your nursing relationship.
Do watch for her growth spurt coming up, she may want to eat more frequently and be extra hungry at night, waking more often.
Pay attention for signs of reflux (see the reflux board) sometimes the off/on nursing and seeming frustrated at the breast can mean other things too.