According to the Oxford Advanced Learner's Dictionary and Cambridge Dictionaries Online, leave is an uncountable noun when it is used to mean "a period of time away from work for a holiday/vacation or for a special reason." Thus, one should say or write,
He's taken sick leave since last week.
However, I found the following sentences both added an 'a' before leave. Are both "without a" and "with a" in front of leave acceptable, even though the dictionaries I've consulted with don't show that leave is singular?
At my request, the board of directors has granted me a medical leave of absence so I can focus on my health. (from Job's letter to Apple Staff as he takes new medical leave of absence)
Steve Jobs' decision to take a medical leave from Apple Inc. was probably triggered either by an infection, a rejection episode related to his recent liver transplant or, most likely, a recurrence of his pancreatic cancer, experts said Monday. (from The Los Angeles Times) P.S. One can find the usage, "to take a week's/a month's leave", in dictionaries. But, I don't think it is relevant to my question. It seems just another usage.