They mean "low enough to take you off the meds." That usually happens when your pressures start plummeting and you get lightheaded while on the meds.
1) I don't know of any correlation with the length of time that you are sick. It makes intuitive sense that the more damage, the longer the repair takes - but immensity of damage is not tightly linked to duration of acute illness. Some people are sick for months but feel better the moment the placenta is out. Some people crash during SNL and are in ICU for three weeks. So it's possible for the disease to cause a lot of damage in a short time or only mild damage over a long period.
2) I'm chronic but not medicated because my bp has been low-normal since 4 months postpartum. In my family history hypertension hits during menopause. I'll report back in a decade or so, 'cause I'm not yet 40.
3) I'm convinced the answer to this is that every pregnancy is different, because every placenta is different. It's the combination of maternal and paternal genes, and that varies with every pregnancy. Some combinations (of partners) make preeclampsia more likely.