Depends on the specimen, if it's cytology like pap smears and other fluids, may take more than 15 min. If it's something as benign as appendicitis, less than 5 mins.
I can't really gauge the time but it's more along the lines of seeing it many times subjectively makes me feel the time flies faster. I know some pap smears take less than 5 to 10 minutes. Some institutions would have a cytotechnician to do those reading but ours don't so we get swamped from those on top of the tissue biopsies.
The longest slides are those from rare specimens like brain tissue, salivary, and pancreas. Other than reading the slide, one needs extra reading from the current guidelines to diagnose which has their own specifications and it's easy to get lost track per organ.