How to reassign a task to group one hour after the user just accepts the task and do nothing?

I'm not sure that we can achieve this within the task settings directly, for specifically referencing task acceptance time.  As Stefan notes, escalations and exceptions are based on task assignment time.

You could potentially solve this with a side flow in the process model that checks a Task Report, one of the data points available is Net Work Time (=NetWork()), which will show the elapsed time between acceptance and current time for active tasks.  Some side path would have to check this report for the specific task regularly, and restart the task once the NetWork time grows out of your range.

https://docs.appian.com/suite/help/25.1/Process_and_Report_Data.html#task-metrics

This is from an Active Tasks Report, you can see the rows are blank where the task is not yet accepted:

I'm not sure that we can achieve this within the task settings directly, for specifically referencing task acceptance time.  As Stefan notes, escalations and exceptions are based on task assignment time.

You could potentially solve this with a side flow in the process model that checks a Task Report, one of the data points available is Net Work Time (=NetWork()), which will show the elapsed time between acceptance and current time for active tasks.  Some side path would have to check this report for the specific task regularly, and restart the task once the NetWork time grows out of your range.

https://docs.appian.com/suite/help/25.1/Process_and_Report_Data.html#task-metrics

This is from an Active Tasks Report, you can see the rows are blank where the task is not yet accepted:

I have done this before (though a long time ago) - it's complex enough that it should probably only be implemented when the need is critical (or the dev is super bored), but it does work.