After the dog will hold the sit command for at least 2 seconds between interval rewards, you can immediately introduce the release command, "free". This will mark when the dog has held a command for the required duration. At that point the dog is "free" from command and may be rewarded.
The major difference between duration and interval training is that now, during duration training, the dog must hold the sit position perfectly for a duration of time in order to get the reward at the end of that time. The interval reward in the previous step is much more lenient, as the dog needs only to be in the sit position at the MOMENT the interval reward is available, if not the dog must wait until the next determined interval of time to receive the reward. This encouraged the dog to hold the position better as to not miss a reward. This sets the dog up to easily succeed at the duration requirement.
This is best done by starting with an easy 1 second duration (since the dog has already held for at least 2 seconds between intervals in the previous step). Then, work to longer durations.
To do this, say the release "free" about 1 second after you praise the dog for initially obeying the sit command. Then, immediately lure the dog out of the position with the reward. Be sure to praise again, to reassure the dog's correct action, THEN you give the treat. With each repetition still praise immediately when the dog obeys the sit command, but slowly increase the time from 1 second to several seconds between the praise for initially obeying the command and when you say "free" (which marks the end of the duration). After saying "free", continue to lure out of position, praise, and give the reward in that exact order. Do not give a reward at any other time! If the dog breaks command before waiting for the release "free", start back from the beginning (Dog's name, "sit", "good boy", etc..). If the dog is having trouble holding a longer duration of several seconds, then do more at a shorter duration before building up to longer durations.
You can move to the next step if your dog consistently holds the command, "sit", for a duration of at least several seconds and the dog responds to the spoken release "free" without needing to be lured.
You can see an example of teaching the duration for sit at the 4:49 mark of the video below: