Track estimated and actual time
When I do my daily review a really useful piece of data to have would be the estimated duration for actions. It would remove just a bit of friction from thinking about how long each action may take when I'm figuring out how much I can get done. Also, being able to loop back and enter the actual amount of time would be helpful for helping identify issues with my estimations in general as well as for reviewing if/when similar tasks arise in the future.
It is absolutely part for the GTD system. Next Actions, according to the book, should be engaged using the following criteria: 1. Context 2.Time Needed 3. Energy 4. Priority.
IQTell (which has announced is end) had the ability to sort by all 4 of these. I believe it is essential, when looking at a list of Next Actions, to NOT have to think about how long each one will take when selecting (which may take up most of a 5-10 minute period that could have been productive).
The whole point of GTD is to think ONCE about an action.
Rafael Garcia commented
You could use the notes field for this. Since what you are proposing isn't part of the GTD system, I'd personally prefer that the software not include it.