This will get you the top 100 with the highest average duration since the last service restart. This DMV is reset when SQL Server restarts, so if you need to go beyond that, you'll need to trace, use extended events, use auditing of some kind (built-in or manual), or invest in a 3rd party monitoring tool.
You can adjust that obviously, or go after different metrics such as physical or logical reads...
USE [Your Database Name];
SELECT TOP (100)
WHERE DB_NAME(database_id) = N'Your Database Name'
ORDER BY total_elapsed_time / execution_count DESC;