Learn Finnish: Tarvitsemme arvion viikoksi. - We need the estimate for a week.

The translative ending -ksi is used here to express an intended duration. Saying viikoksi means that the estimate is meant to cover a period of one week, or will be valid "for a week".

Could viikoksi also mean we need the estimate by a certain time?

Yes, the translative is also used for deadlines! However, when using a plain time span like viikoksi (for a week) or vuodeksi (for a year), it usually means intended duration. To express a deadline, you'd typically use it with a specific point in time, like ensi viikoksi (by next week) or perjantaiksi (by Friday).

How would the sentence change if we needed the estimate "within a week"?

To express the time frame in which an action must be completed, you use the inessive case (-ssa/-ssä). So, "We need the estimate within a week" would be Tarvitsemme arvion viikossa.

Why is the object arvion instead of the partitive arviota?