For what length of time can you actually have symptoms of multiple sclerosis before you are diagnosed?

Could be years. Have seen many patients who have clearcut MS, who had delayed diagnoses up to 5-10 yrs after onset of first problems. Many reasons for this, including lack of insurance, denial of disease, fears of physicians, moving from place to place and not completing evaluations, being diagnosed as having psychiatric problems, etc.
Variable. No definite rule. It all depends on what area of central nervous system is involved. Some areas can be silent clinically .