で

About

The で particle can be used to describe how long a given action takes to complete. If で is followed by a past tense verb, it marks how long an action took. If で is followed by a present tense verb, it marks how long an action will take.

E.g.
3時間じかんでやる。
"To do something in three hours."

3時間じかんでやった。
"Something was done in three hours."

Both で and に may be used to mark lengths of time, however, they differ in nuance. In general, に marks the ending point of an action, or the specific time when an action is performed, while で marks the length of time an action takes.

Summary

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  Use the で particle after an amount of time to describe how long something takes/took

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  Compared to に which marks a specific time, で marks a length of time