I have only seen collections run during the day, when archiving isn't occurring. You do not want storage to have to split time between archiving and building cab files.
The length of time depends on where you are in the process. When you first turn it on, it will take awhile to catch up, but once it is in maint. mode it needs less time. It does write to the app log when it starts and completes, so if you monitor that you will get an idea of how much time your environment needs to run.