Long time. Chlamydia is a stealth germ. You can have chlamydia without having any obvious symptoms. This is part of the reason that pregnant women are routinely screened. Other women who should be screened: sexually active 15-25 year olds, women>25 year old with new partners or multiple partners.
Unknown? I couldn't remember if there has been any study done to document this concern. But common sense seems suggesting it may be dormant forever. Theoretically, blood test can be done, but usually, it's not clinically necessary. Probably a proposition of money-wasting. But the longer symptom-free, the better and safer she will be. Nonetheless, all sexual partners need to be counseled and treated.