Providing a leave of absence is one way to "reasonably accommodate" disabled employees. But the Americans with Disabilities Act (ADA) doesn't require you to wait forever for an employee's medical condition to improve to a point that would allow the person to return to work. It's not meant to be an open-ended demand.
Where can you draw the line? Courts have established rules-of-thumb about how much leave you must accommodate.
Recent case: County court worker Mark Wood suffered from cluster headaches, which caused him to miss work frequently, often one to two months at a time. To accommodate his condition, the county granted many leaves of absence without pay. But when he continued to miss nearly five months of work during one year, the county fired him. He filed an ADA suit, but an appeals court sided with the county.
The court's reasoning: Wood was not covered under ADA because he couldn't perform the essential functions of the job, with or without a reasonable accommodation. Wood argued that a leave of absence was a reasonable accommodation. But a leave of absence with no foreseeable end date wasn't reasonable. (Wood v. Green, No. 02-12971, 11th Cir., 2003)