The Question and Answer section for Kindred is a great resource to ask questions, find answers, and discuss the novel.
In the section entitled, The Fight, Dana has returned home after two months in the past. After eight days in her home, she returns to Rufus' time.
I don't think Dana chooses when her trips happen. She just gets dizzy and wakes up at some point in history.
Rufus is Dana's ancestor, son of Tom Weylin and heir to the plantation, master of Alice, father of Joe and Hagar.