To reset a challenge mode (and you'll be doing this a lot), you can right-click your character portrait and choose Reset Challenge from the menu, or just use this macro:
For the first time you do every challenge, it's generally best not to reset for every mistake (which is something we did and regretted). Do the dungeon from beginning to end no matter how many wipes it takes. Learn the pulls, identify where you'll be using invisibility potions, and complete the objectives. A group that keeps pecking at a dungeon can manage all bronzes, and probably even silvers, without doing anything special.
How quickly can we finish all 9? Progress will depend on how prepared your group is and how quickly people get used to the CC and cooldown requirements that allow a group to move briskly. As always, the Schedule Boss is an omnipresent concern. Great is she, and terrible as the dawn.
My group worked on the weekends on and off for about a month. A plurality of our time was spent in the first two dungeons we did, which were Gate and Scholomance. Once we'd gotten gold in these two (and whenever we could get everyone online at the same time), the group knew what to expect and golds started raining from the skies. Experienced groups running challenge modes for valor or a paying customer can knock out all 9 golds in around three hours.
On the subject of bad nights: Sometimes due to a combination of circumstances beyond your control (someone doesn't show up, computers go boom, a key member keeps lagging, sunspots, each and every single one of you has by unspoken agreement decided to suck all at once), you will have an awful night. Just walk away and come back later. Banging your head against a wall won't do any good, and things usually turn out better when people have had some time to think about how to improve a run. Take a break, watch some videos, and come back for another try later.