Turning on OCR for a document can be done in the "Document properties" pane. To access this, right click your document and select "Properties".
With the document properties panel open you will find the OCR toggle.
The time it takes to turn on OCR will vary, depending on the document size. You will see this progress indicator while OCR is being applied.
A small magnifying glass icon will appear next to the document icon when OCR is applied.