Possible Cause 1: Foremilk/hindmilk imbalance
When you start a breastfeeding session the first milk that comes out is very watery and thirst quenching. As you continue to nurse the milk becomes increasingly more fatty and appetite quenching. If your wife suffers from an oversupply of milk or switches sides during a nursing session too frequently, the baby will get too much watery foremilk and thus not feel satiated and want to nurse longer to stimulate supply. It is best to alternate breasts no more frequently than approximately every 2-3 hours (if she has a severe case of oversupply this may increase to as much as switching sides every 6 hours), to make sure the baby completely empties the breast and gets a solid helping of higher-fat hindmilk. If the baby is still hungry after completely emptying one breast it is, of course, ok to offer the other after.
Possible Cause 2: Nipple confusion
Sometimes babies who are offered bottles in addition to the breast develop a "preference" for bottles because they don't have to work by sucking to get the milk, and will sit there not sucking and swallowing while nursing. Solutions would include eliminating bottles, or selecting a bottle with an extra slow-flow nipple or two-stage nipple that most mimics the effort required to breastfeed.
Possible Cause 3: Growth Spurt
From the age you've listed, your daughter is probably just coming out of her 6 week growth spurt. Growth spurts come and go, and babies will typically want to nurse what seems to be constantly for about a week. So it's likely what you've been seeing in the last week or two is above average and will soon subside. Letting her nurse as long as your wife can stand it is the best thing you can do as your supply adjusts a couple days in and they can take in extra calories.
Possible Cause 4: Comfort Nursing
Nursing is comforting for babies. Sometimes they nurse when they aren't even hungry, just because it's comforting. It's a lot easier to identify this type of nursing in older babies because the difference is more pronounced, but basically they stop sucking hard and swallowing and just suck lightly without swallowing sounds. If the amount of comfort nursing is more than mommy can handle, a pacifier, used in moderation, is a good solution, however be cautious not to offer the pacifier when the baby really is just hungry and needs feeding. Compressing the breast to express a little extra milk in their mouth to encourage them to swallow more may also help. If the baby isn't really hungry, they aren't going to be efficient about eating...offering to eat later when they're more hungry may get better results.
Possible Cause 5: Baby is falling asleep
Babies like to nurse to sleep. If the baby is sleeping, it is ok to unlatch the baby and put them down for a nap. Healthy six week old babies will wake up to nurse when they get hungry enough, and take frequent naps. If the baby genuinely needs to eat and can't stay awake well for any of her feedings and needs help staying awake to feed, you may want to try additional techniques such as lowing the temperature controls, removing some clothing or blankets from the baby, gentle rubbing or tickling, dimmed lighting and non-stimulating environment while nursing, wiping with a dampened cloth, etc. (additional techniques: http://www.breastfeedingbasics.com/articles/waking-a-sleepy-baby )
Possible Cause 6: Baby is perfectly normal
6 week old babies typically nurse every 2-3 hours, 8-10 times a day or more, with nursing sessions that may last 30-60 minutes. Breastmilk is digested quicker than formula, so breastfed babies need to eat more frequently, on demand rather than on a schedule. Some babies like to cluster-feed and have several of their daily feedings close together throughout the day, more frequently than every 2 hours. Prepare a snack station for mommy next to her favorite nursing spot with snacks and beverages that are easy to eat with one hand, put on a movie or music to keep mommy from getting too bored, and get comfy. And keep in mind, this too will pass! As they get older they become more efficient at nursing and will only take 10-15 minutes instead of 30-60 for a feeding, and will feed less times a day. There's really nothing you can do to teach them to nurse more efficiently, they pick it up on their own.