I saw on other service that users completed tasks will be deleted after 1 week if they don`t has premium account. Yours service do the same?
In Todoist, you can access the completed tasks in each project anytime and they will never be deleted unless you click on the "Delete completed" link at the bottom.
After a while, you may need to click on "Get completed tasks" multiple times to load all the data, though.
With Todoist Premium, you get an additional overview of your completed tasks which would stay even if you click "Delete completed" in a project and shows tasks completed across all projects, but the basic completed view in a project is available for free.
What type of this "Additional Overview" premium users can get?
You can view ALL completed tasks, you can sort them in a few ways, such as by project, date, or label. You also get nice charts that show how and where you completed your tasks. All accessible from the Karma score at the top right corner.