Once you understand why you need to rake leaves, the question becomes one of when to rake leaves off the grass.
Determining how long you can let leaves stay on the lawn before raking them off involves a judgment call: one can't really put a number on it. The tip below can be used as a general guideline so that novices to lawn care can at least become aware of what factors need to be taken into consideration.
Waiting Three or Four Days Before Raking Is Okay
A general consensus is that leaving leaves on the grass for more than three or four days may be unwise. That said, other factors must be taken into account, too. The thicker the layer and/or the wetter the leaves, the sooner you should rake or otherwise remove them, because these factors increase the likelihood of the grass underneath being smothered, etc.
How many leaves you anticipate having to rake up should also be weighed as you determine when to rake leaves. Large yards with a lot of deciduous trees obviously pose a bigger cleanup challenge than other yards. People who own such yards may have to start raking earlier, just to ensure that they do not fall behind in what will be, for them, a big lawn-cleanup project.
Another factor that might influence your decision on when to pick up leaves is the leaf-removal equipment that you will be using. If you use a leaf blower properly, you may be able to pick up the leaves faster than you would using a rake, allowing you to procrastinate a bit longer, but using leaf blowers is not for everyone. Other people run a lawnmower over the leaves, catching them in a bag attachment, but that is not the only way to incorporate a mower into this project:
Nutrients in Leaves
One time-saving method for leaf removal that may preclude having bag, blow, or rake leaves is to use a mulching mower, instead. A mulching mower will break down the leaves sufficiently to allow you to let them stay on the lawn. They will harmlessly work their way down into the soil, eventually. Using this method, you are essentially treating the processed leaves as a lawn fertilizer. The argument in favor of doing so is the same as for letting grass clippings remain on your lawn, especially if you chop them up first using a mulching mower.
Whether you have decided to let the mulched leaves stay on your lawn or remove them, do not waste this wonderful source of free organic matter; shredded leaves are invaluable if you make your own compost.