If you have 6 major locations and each location should have a dozen quests, you need more than one writer
How much time it usually takes to write one decent, average sized quest for a writer who can use his tools well and has a general idea for the quest he is doing?
Depends on the complexity of your choices (the more different paths it offers, the longer it takes) and how many ancillary writing things you need to do (not just dialogue but also in-game notes and books, journal entries, etc).
Depending on how quickly you write, if you have already planned the quest out it takes between 2 days and a week to implement it.
Keep in mind you also have to work with the coders and level designers (though ideally, during your time at the company you have learned some basic skills with the scripting engine and level editor to do basic scripting and level implementation yourself).
A rough estimate on my own experience which is of course not representative of every writer's approach and every company's workflow:
First, a basic quest idea is developed
Then, you take a day brainstorming the quest and discussing it with the other writers, getting feedback on the quest and asking the other writers if anything is missing (maybe you haven't thought about a rogue option yet, but you already got fighter, mage and cleric approaches covered). This takes maybe a day. Could take longer depending on your position in the team: if you're the lead writer, you already know where you want the quest to be within the big picture and you know how to make it fit; when you're not the lead writer you gotta talk to the lead and coordinate with others who might be working in the same area. This is why it's a good idea to assign writers to different areas, btw. Let one questline be done by one writer, not every quest in a questline by another. Let one location be done by one writer, not every quest in the location by another.
Let's say you have a town where the lead writer himself is contributing 3 quests, and two other writers are contributing two quests each. Coordination and discussion of the quests might take up to three days to make sure they all fit together and have some interplay with each other (it's always cool if one character has relevance for more than just a single quest). But not much longer than that.
Then, when you know where you want the quest to go, what kinds of choices the player should have, and what kinds of things can affect dialogue choices (skill checks, race and gender, class, maybe even equipment, etc), you start writing the dialogue trees. If you already know where you're going (which is why you should take enough time during the brainstorming session), this can be done within a day, two at max.
Then you write ancillary texts like journal entires, which shouldn't take you more than a couple hours if everything else is already done.
Overall, a week per quest is a realistic timeframe for a writer who knows what he wants to do.
But keep in mind you also have to (or, well,
should because it's often not the case) work together with the level designers and scripters for optimal results, which might add some more time of in-team coordination to it.
If you've read Swen's recent report on the development of Original Sin 2, you may have noticed that he said a lot of the writers they hired had no experience with non-linear dialogue trees, and sometimes broke the scripting. Fact is that often, companies hire outside writers to help in a project, and these are harder to coordinate with the team than in-house ones.
You absolutely
should make your writers coordinate with the rest of the team to prevent such situations. At the very least, they should know what can be done with your scripting engine so they know the possibilities and limitations of what they can and can't do in quests.
Lack of coordination and direction is the #1 reason why some projects gets bogged down imo. Additional time spent coordinating between team members is time saved in the long run because there'll be less issues to fix later on.