The ADA specifically provides that time out of work can be a reasonable accommodation. Note that this is particularly useful when you have a combined right to FMLA and ADA leave. If you have a medical issue that counts as both a serious medical condition under the FMLA and a disability under the ADA and run out of twelve weeks of FMLA leave, you may be protected by the ADA if you need to extend that leave. But, for this to stick, the leave must be of a reasonable duration. At the very least, this means that the leave must be for a definite period. For instance, assuming that your job is not a crucial one for your workplace, you are probably safe if you have a disability and a doctor’s recommendation that you should stay out of work for a month. The further you push out that time frame, the more likely you are to risk that your employer will claim that continued leave is unreasonable and have a court agree.
You can read more about this issue in chapter 9. For detailed discussion on the law in this area, check out an article by attorney Ramit Mizrahi, “Leave As A Reasonable Accommodation Under The Americans With Disabilities Act,” 3 LABOR & EMPLOYMENT LAW FORUM 29 (2013). You can find it online at http://digitalcommons.wcl.american.edu/cgi/viewcontent.cgi?article=1055&context=lelb.
If you find yourself facing a possible need for extended leave due to a disability, see a lawyer to find out how courts in your area deal with this issue.