To express a stretch of time subsequent to the action of the main verb, use на plus Accusative. This contrasts with the Accusative used without a preposition, which expresses duration simultaneous with the action of the main verb:
Ї пољхал тудђ на недљлю. I went there for a week (with the intention of staying a week).
Ї љхал недљлю. I travelled [for] a week (I spent a week in transit).
You can't make a one-to-one association of Russian на with English for, because you can omit the word for in the second example above, but not in the first.