Depends on the cover/cover lifter. Anywhere from about 9" to 18" or so.
I would ask your dealer what they recommend for the cover they are supplying.
The cover lifer my dealer was including would have required about 18" which wouldn't work well for my install but they gave me the option to change it for the 9" clearance one. What the difference was is the low clearance lifter keeps the cover up at tub level, which does add privacy but does block the view out of the slider. I decided that trade off was worth it for my space. Your space might be different.