I'm not Mike Lardy (and is that an understatement). The answer is that it depends. If a seven month old dog comes to me with no previous training, I plan on a month for ob. OB isn't the only thing I do but that is the focus. Note that it takes what it takes but best to do a good job before moving on.
In the TRT DVDs, Mike will often say something like usually spend a couple days on this, two or three days on that, but its not as simple as trying to say your dof will be ready in x days. As with every step in the program, you spend as much time as needed to be able to do the next step without having to deal with problems from a previous step. If you get in to FF (which is hold, ear pinch, walking fetch, and stick fetch) and you are having to deal with obedience problems, it will prevent you from progressing.
A benefit of following the program is that you will identify holes in a step during the next step. So if your dog will not walk properly at heel, you will see this when trying to do walking fetch. Don't be afraid to move forward, but also don't be afraid to back up.
I was taught when you think the dog is doing the drill correctly with no corrections do it for 3 more days to allow the dog to enjoy the success of the work...then move on ...as captainjack said if you move on and have difficulty with something from the past, you moved too soon....The dog will never be more reliant then your are thorough...Steve S
Consider force fetch to be part of obedience. It is one of the tools that is added after pup gets his permanent teeth. The other obedience commands can be accomplished prior to the permanent teeth eruptions.