I poked around a bit on some of the links suggested by La Leche League, well-known experts on breast-feeding, and found some information that may answer your underlying question.
On their "is baby getting enough milk?" resources page, I found a link to a useful description of the baby's eating process. One part in particular sounded very familiar to me, and I'm sure I've seen it in other sources:
those early, quick sucks get everything arranged in his mouth and alert your breast to release the milk. Once your milk is flowing, your baby can’t suck nearly as fast. That change in the rhythm shows that your baby is taking milk. With each swallow, you’ll hear a whispered “kuh” sound –more of a glug for some babies, very quiet for others. Most babies swallow with every suck or two during this phase. Active feeding is a slower rhythm than“priming the pump”.
So a good way to tell when your baby is done feeding may very well be to just listen for a change in the sound. Different babies have different eating patterns, and the volume of milk you produce should increase over time, so timing the feedings probably won't be very effective.
That same link also provided this advice, and while I find the animal comparisons a bit off-putting (I know my wife wouldn't appreciate being compared to a gorilla, dog or a horse!), I think the basic concept is sound:
How do I know when my baby’s finished? He may just drop off by himself, relaxed and loose-limbed. But it doesn’t really matter! Think dog, or horse, or gorilla. If a gorilla Mama can’t reach that banana with the baby attached, she just takes him off. She doesn’t worry about whether or not he’s full. If he fusses, she puts him back on after she gets the banana.
So listen for the sound of your baby swallowing (I've found anecdotal reports that not everyone can hear this, though, so don't worry if you can't tell just by listening), and when you think she's done, you can let her suck longer to self-pacify if you'd like, or you can try unlatching her and seeing if she is content to stop.