The sun is preparing to set in this picture. That can happen surprisingly fast, as you know, but at the same time you must wait. That is the pace at which to start to imagine how long it might take to proofread or edit something.
Many factors are involved. For example, the experience of the proofer/editor, the nature of the piece to be reviewed, whether you want copyediting or proofreading, the amount of text and number of pages, if the piece is a text document or if the piece is already laid out into a design, etc.
To help reduce the time spent proofreading or copyediting, provide any background information about the item to be reviewed, such as what it will be used for, if it’s a printed piece or an online piece, or if it’s part of an existing campaign. If there are things you don’t want checked or modified in any way, mention them.
In other words, a “small” piece with just three words doesn’t take two seconds to review. Allow at least 30 minutes.
The answer: Anywhere from 30 minutes to several hours. With a quick glance at the material and with any background information you provide, a good copy editor or proofreader should be able to estimate the time needed.