Note: This is with the new relationship clickapp which is in beta.
When creating a rollup custom field that calculates time estimation for the Tasks relation it seems like it isn't able to calculate time from subtasks in the related tasks.
To reproduce:
  1. Make sure that the Relationship clickapp and the Time rollup in subtasks clickapps are enabled!
  2. Make a task with subtasks where you estimate some amount of time in the subtasks (Time is correctly calculated in this task)
  3. Create a new list with a rollup custom field with the following settings. "Related to: Tasks" and "Rollup field: Time estimate" and make sure that the calculation mode is "Sum".
  4. Create a new task and link it to the main task that we made in step 2.
  5. Go back to the main view and it should be "0 m"
(Note: The surprising thing is that it seems to work when i add the rollup field inside the task itself, but not in the list view.)
I have attached some images, but i've had to blur them. If you need a base example, then i can make some that are not blurred :) (I could probably create a template maybe?)