Hi Maria‚ - it depends on the settings you have for timesheets. The admin can select the timeframe for how long tasks/projects stay in the timesheet.
We have ours set for 3 weeks so tasks and projects stay on the timesheet for 3 weeks based on the projected completion date.
This article explains it (probably better than I can), but here is a snippet from the article.
Hope this helps!
Erica