I worked for FWS in Alaska, so somewhat different agency, but somewhat similar aviation uses. So, to answer your question, i can only speak for what I saw in FWS, but I suspect NPS "full time" pilots (as in pilots with no other official duties) wind up kind of in the same boat as those in FWS. Requirements/qualifications are same, etc.
What I saw with "full time" pilots in FWS regarding what "Other Duties-note there is that statement in the position description, was that it totally depended on the particular pilot and their supervisor. Frankly, a lot of supervisors in government jobs aren't very motivated to actually "supervise" their employees. So, a go getter type pilot can and will find work to be done. On the other hand, a pilot who feels all he or she is REQUIRED to do is fly......is very likely to be a problem employee. I've seen more than a few of those folks either resign or be fired as a result.
But, if you're a team player, and motivated to stay busy, most any organization has room for "helpers", in a variety of roles. May be anything from assisting with tagging critters to shopping for groceries......I was a full time wildlife biologist/pilot and believe me, I've shopped for a LOT of groceries for my field crews. Point is, there's ALWAYS work to be done, if you're not too proud to do it. I've seen full time pilots succeed by being "that guy", who, when a project happens, he's first in line saying "how can I help?".
Flight times. I was blessed by VERY understanding bosses over the course of years. My annual flight times at work ranged from 300 to a high one year of 700 hours, when we had a REALLY rocking biological program going. But, I flew for most if not all the FWS offices in Interior Alaska and some of the others. I also did some flying for NPS on surveys during those years, including at Denali. My bosses figured it made them look good that they'd share airplanes and pilot to other programs. The only caveat was MY work had to be done, on time. And, BTW, at GS-12, there is no overtime authorized. Comp time is pretty limited (has to be used in next couple pay periods), so I donated a LOT of time.
But, I LOVED that job, and I'd do it all again.
So, the key to happiness, as in most jobs, can be found within oneself. Having a really good boss sure doesn't hurt, though.
Also, understand I retired fifteen years ago, so keep that in mind.....agencies change, projects change, etc.
MTV