Parts of the form will need to be filled out by your physician, if this is a "typical" claim form.
On your part - be honest. You don't know when you can return, just state "unknown." You're not dealing with a broken leg that the doc says will need 6 months rehab. Lupus and other autoimmune conditions are unpredictable. It could be you'll be off 6 months and then return with a smile on your face. it could be that, like our librarian's aunt, you'll be off a longer period of time.
After granting your leave (LTD is easier to get than Social Security Disability so I'm being positive!) the company will probably have you recertify every so often - possibly every couple months at first, then if your condition keeps you away from work for a longer period, they'll probably switch to yearly.
At some point, they may require you to apply for Social Security Disability Insurance - remember that the guidelines for SSDI are FAR more stringent than for long term disability.
Best wishes, and hope you'll get approved so you can get the rest you need - AND get back to work in a few months if that's your goal!
~"I find hope in the darkest of days, and focus in the brightest. I do not judge the universe." (Dalai Lama)