It depends, but generally speaking you don't need to empty the bin after every use. Many robot vacuums can empty themselves from their docking station, so you don't need to clean the actual robot.
Self-emptying robot vacuums can collect 30 to 60 days' worth of dust and allergens, making cleaning a breeze.
Other robot vacuums don't offer a self-emptying feature, but they are cheaper. Depending on how much dirt and pet hair you have on your floors, you can use them a few times before you have to empty the bin.
It's a good idea to read the product description. It will tell you whether it offers a self-cleaning option and how long the bin will last between uses.