Of course we don't know the answer to this today. I think we entered the year with a couple of fair thoughts. First, this was the most talented and reliable collection of arms the Nats had started a year with perhaps during the whole window. Second, there was still a measure of variability in these arms beyond the usual "Hey, it's the pen! they don't pitch much!" kind of variability.
The first sounds great but it also speaks to the general questions the Nats have usually allowed the bullpen to start with. This isn't an "A" pen right now. Looking at it it's a "B" pen with potential. But the Nats have been content to start with "C" pens and "B" pens with little potential, so this becomes a high water marks. The latter speaks to the fact that while all these arms looked good on the mound last year there are fair questions to be asked of each and every one. What would not be a question? A healthy young arm that has pitched well in the majors the past two years. Well Treinen has pitched well for one. Kelley is questionably healthy. Blanton is old. Glover has little experience. Each one of these have a lacking quality.
This doesn't mean the pen is bad. Like I said "B". It doesn't mean they can't be great. But it does mean it's fine to have concerns about them and to look to the beginning of the year to allay/confirm those concerns.
How long should we wait? I'm looking at about 12 normal situations for each pitcher. Don't ask me why 12. 10 seemed a little short. 15 a little long. 12 would be about a month of usage a tad on the heavy side but just a tad.
What should we want to see? Here's a little quiz. What do you think the average save percentage was in the majors last year? No. Lower. No. Lower
If that seems low remember that you can have multiple blown saves in a game but only one good one. So it's not a great stat for seeing what a pen should do but you could use it to see how the Nats are looking compared to average. Another thing that could be done is looking at just the save leaders and see how they do. In general an average year for a good reliever will have about a 90% save percentage. So you should do your job about 9 of every 10 times out. You can give a little leeway for those in middle relief. Maybe say... 10 of 12? If you are around 9 of 12 that's getting to be not good enough. 11 of 12 and that's great.
It's a rough idea. Come up with your own if you like but that's where I am. I'll update these stats as the year goes on and see how it works and how it matches up with the "feels" and with what the team is doing.