If you're like many people, you probably wait to empty your vacuum until the contents of the cup or bag reaches the fill line. If you want maximum performance from your vacuum, though, you should empty often.
The informal test on bagless vacuums
Any amount of dirt and lint in the collection apparatus of your vacuum can hinder its performance, according to some online sources. I wasn't so sure, so I gave it an informal test with two different vacuums.
First, I tried picking up a dime-size piece of paper with the Dyson V6 Absolute. When the cup was empty the paper was sucked up immediately. When the cup was one-third full, the paper took longer to be sucked up. It wasn't long, but it was noticeable.
Next, I tried the same test with the Bissel Powerforce Compact and got the same results. An empty cup always provided better suction.
What about bags?
The Carpet and Rug Institute recommends replacing or emptying vacuum bags when they are half to two-thirds full. This may seem like a waste of a good bag if you use disposable bags, but it does make a difference and can make your vacuum last longer.