I believe the Dungeoneering skill goes up to 120.
The longest time it takes to get to 99 Dungeoneering depends on your combat level, as you go deeper in Daemonheim, the monster will be more powerful. Depending on you and your team mates levels the monsters/bosses levels will change also.
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It depends how long you are on Runescape.
This entirely depends on what level you are starting at and what you are using to train the skill.
about 70-100 days when you have got the level for nature runes
That Entirely depends on how many hours a day you play runescape.
It depends on the average hours you spend wc'ing a day.