While the show glosses over it, there's a difference between time travel and parallel universe creation, and it comes down to how many "copies" of a person there are:
In time travel, there's only one "copy" of an individual. That person somehow exits the normal flow of time and enters back in a different place.
In Voyager, this includes episodes like "Parallax", "Eye of the Needle", "Death Wish", "Future's End", "Before and After", "Fury", and "Shattered".
In parallel universe creation, there are multiple "copies". An event spawns a copy of the existing universe at a specific moment of time, creating a copy of the individuals within it. In usual Star Trek fashion, that copy is usually either destroyed or never heard from again when the events of the episode resolve and the viewer's frame of reference goes back to the original Universe.
In Voyager, examples of this include "Time and Again", "Deadlock", and "Year of Hell".
So, looking at the proper time travel episodes:
"Parallax"
In this episode, the crew is trapped inside a quantum singularity. Janeway and Torres take a shuttle at leaves Voyager's frame of reference, but is resolved to be only a 20 minute discrepancy.
Janeway's cumulative age differential: 20 minutes
"Eye of the Needle"
Janeway doesn't time travel in this episode: the time traveling is done by a Romulan through a wormhole.
Janeway's cumulative age differential: (still) 20 minutes
"Death Wish"
In this episode, the Q are Q and use their powers to throw Voyager back and forth through time, but only for a few minutes, and the viewer sees this in real-time.
Janeway's cumulative age differential: 25—30 minutes
"Future's End"
In this episode, Voyager is sent back to the 90s and is there for at least a few days: watching the episode, I counted 2 day and night cycles. Once the events resolve, they are placed back to exactly the same moment they started at.
Janeway's cumulative age differential: 2–3 days
"Before and After"
Only Kes is a time-traveler in these episodes.
Janeway's cumulative age differential: (still) 2–3 days
Relativity
Janeway never sleeps, nor suffers from sleep withdrawal, nor is there any mention of any significant length time elapsing during the events of this episode. This suggests that the events were only a few hours, if that, but let's be conservative and say a full day.
Janeway's cumulative age differential: 3–4 days
"Fury"
Only Kes is a time-traveler in these episodes.
Janeway's cumulative age differential: (still) 3–4 days
"Shattered"
A copy of Janeway does time travel, but not the main timeline version: only the main timeline Chakotay time travels.
Janeway's cumulative age differential: (still) 3–4 days
You've mentioned a few other examples of temporal anomalies, but none of them significantly affected Janeway's proper age:
"Blink of an Eye": even trapped in the gravity well of the tachyon planet, Voyager existed outside its frame of reference. They could not travel to the surface without dying (sending the Doctor instead), and the inhabitants of the planet could only travel to Voyager with special temporal compensators (once disabled, they died instantly).
"Time and Again": As you suggested, the timeline is reset. As is common in Star Trek, an event created a parallel universe outside the original timeline's frame of reference. The Janeway that existed in that universe does not exist in the main one, so when its events resolve, it goes back to Janeway Proper, who hasn't aged at all and isn't replaced by the parallel Janeway, who has aged. Compare this to "Deadlock", where:
Harry Kim is killed, and a parallel Harry Kim takes his place.
So, when its all said and done, Janeway is only 3–4 days older than she would've been had she not time traveled, with most of that coming from the "Future's End" excursion.
But how old is she? We don't know. According to a callout on the Memory Alpha article for Kathryn Janeway:
According to an okudagram shown in "The Killing Game", she was born in 2344, however, this would mean she was only 27 in 2371 when she took command of the USS Voyager. For comparison, Mulgrew was 39 when she took the role. Not having played tennis for nineteen years since high school in 2373, Janeway was probably around the age of 35 when Voyager's mission began, placing her actual year of birth closer to 2336.
An okudagram biography on the video game Starship Creator Warp II states her birth date as 2332.
Kate Mulgrew stated in an interview on The Late Late Show with Craig Kilborn (18 May 2001) that Admiral Janeway was 76 in "Endgame". It had taken her 23 years to return to Earth and they were celebrating the ten year anniversary at the beginning of the episode, making the year 2404, which puts her year of birth in 2328.
Which, if considered canon, puts the age of Janeway in the main timeline at the end of "Endgame" to be anywhere from 34 to 50, with more likelihood—given Mulgrew's age—towards the upper end of that range. Plus 3–4 days and a few minutes, of course.
