Does kaposi sarcoma develop over time or do marks appear all at once?

Over time. Lesion develop slowly over months and continue to develop over a period of years.
Kaposi. They develop over time. If you think you may have it you should see an md immediately. It can be a sign of advanced HIV infection and is not something to ignore. You may or may not have it because bruises can look the same. The only way to know is to see an md and if you are worried about this you need to go to the dr!