Use Accusative to express how long a time.
The duration of an on-going activity, expressed by an Imperfective verb, is rendered by the Accusative of the noun for the time period.
If the noun is in construction with a quantity word, then it is the quantity word that is Accusative, and the Genitive rules apply (Genitive Singular with двђ, двљ, трќ, четІре and Genitive Plural with the higher numerals and quantity words like мнћго, мђло, скћлько, etc.).
You can use Кђк дћлго? to ask how long something went on, or, better yet, use a yes/no question, like this:
Онђ дћлго жилђ тђм? Did she live there for a long time?
Онђ жилђ тђм недљлю. She lived there a week.
Онђ жилђ тђм однџ недљлю. She lived there one week.
Онђ жилђ тђм двљ недљли. She lived there two weeks.
Онђ жилђ тђм пїть недљль. She lived there five weeks.
Онђ жилђ тђм мнћго недљль. She lived there many weeks.