I know you can set a day and time in todoist but is there a way to set a duration for example
Tuesday @ 0900 - 10:00
Unfortunately, it's not possible to set a task duration at Todoist right now, but we may consider adding such option in the future and you're welcome to vote for it in our Votebox: http://todoist.com/Vote/showProposal/259/
For now, as a workaround, you can plan how long a task would take using labels. For example, you can type @1h or @2h within the task's name as a reference which will also allow you to search for all tasks with an "1hour" label across all projects.
it is mainly for the calendar as it would be a handy feature
Absolutely agree - knowing task durations and seeing them on calendar would allow to do workable plan for a day.