If you qualify for FMLA leave, it is only for up to 12 weeks, maximum. And you will need a doctor to certify that you are seriously ill and unable to perform your job. Generally, there is no law that requires employers to allow employees to take time off for as long as you'd like. The only way this may happen, is if you can make a legitimate workers' compensation claim, by being able to show you have suffered a work related injury or illness. If you think this is possible, you should consult with a workers' compensation attorney.
Mr. Kirschbaum's advice is sound. Generally, you only have a right to reinstatement under the FMLA if you return to work upon the conclusion of your FMLA leave. If you take extended leave beyond the twelve weeks of FMLA leave, you would be well advised to ask the employer, in advance, to confirm that you will be reinstated upon its conclusion.
Disclaimer: This answer does not establish an attorney-client relationship or constitute legal advice. It is for general informational purposes only.