My groups (I have an IRL group and an online group) tend to average 1-2 months for each book.
Regarding the Ice based casters... so long as they have spells to use when up against things resistant/immune to Cold, they should be okay. Just keep in mind that there are going to be a *lot* of things resistant or immune to Cold.
Ice is going to be pretty badly compromised as a damage source. Conversely, fire is pretty awesome, although a fair amount of creatures realize their own weaknesses and takes steps to counter such an obvious issue. That said, having ice based casters could open up a lot of potential social/rp solutions to problems. You could also use a lot of ice/cold magic as battlefield control. Wall of ice, grease spells reflavored as that wonderfully refrozen snow that makes you slip ALL THE TIME, fog clouds, etc.
As for clearing books, that REALLY depends on your group's play style. Many of the dungeons have "optional" rooms, that you don't need to defeat the enemy or even go into the room to finish the adventure. A group that tends to want dungeons "cleared" will add possibly 20-50% more playtime. There are patches with large amounts of overland travel, and the random encounter rates are rather brutal. Iobaria in 3rd book being pretty nasty: 30% every 2 hours! Even if you only do that for when they are traveling, thats four checks. Odds are good of one or even two encounters each day. If you check for every 2 hours, thats 12 chances, average of 4 encounters during the day, including one at night, which can destroy arcane casters who need rest to regain spells. And these all add extra time to the adventure.
Groups who roleplay more might take more time as well. Reign of Winter has a lot of fun and interesting role-playing encounters, and room for even more if you let them as a DM.
From personal experience, my group cleared the first book in five sessions of 5 hours average each. Second book took about 7 sessions. Third book should wrap up with its eighth session. But my group does all the things I warned about: lots of RP, full checks for random encounters, don't like leaving unexplored corners. Your mileage may vary.
Lots of the monsters are cold subtype so they are immune to cold damage but open to a little bit of being manipulated by their cold subtype. Getting cold resistance from their theme will work well and there are ways to work in cold themed characters into the ongoing winter plot.
In my game the white dragon bloodline sorcerer benefitted from his cold resistance.
Witches with winter patrons have a good hook into the story background.
For my group AP's take about 30 hours of game time each. We do some role playing but are not heavy role players.