UPDATE: Feline Familiar will take MUCH less time than I thought it would! Each pumpkin (marked with a yellow exclamation above it) inside a friendly inn that you visit will provide at least 2 tricky treats each (upon your first visit only). Add that to your first Headless Horseman kill for each day, and the quests/dailies that are available, and you can expect to obtain the newest kitty in one to two days (with some patience, dedication and time of course :P)! YAY!
Since I don't want to confuse anyone, the original (and ultimately inaccurate) post can be found after the jump!
Disclaimer: Since I can't test any of this out, I'm going to have to wait until next week to confirm/deny it all. Blizzard may have added more dailies, changed the rewards, or maybe added in other ways to earn Tricky Treats. In any case, this is all just speculation for now.
Being a curious collector and slightly obsessive about knowing what's to come beforehand, I decided to try and calculate how long it will take to purchase the Feline Familiar IF the currency and amount shown on the official WoW armory are correct. If it's not accurate and Blizzard has changed the rewards/currency but has yet to update the armory with the new information, then I guess I'll just have to settle with being surprised. (Booo :P)
After taking a look at all the quests and dailies that reward the Tricky Treats, I've come to the conclusion that you can earn about 22 (give or take a few) treats each day. The Feline Familiar will cost 150 (according to the armory), so this means it will take about 7 days (at least) before you have enough to purchase the cute kitty. It might take less if you get lucky and snag some Tricky Treats from the holiday bags and/or candy buckets!
Hallow's End lasts for 14 days total, but that's including the Tuesday just as it goes live (before the daily reset). Without that day, it's 13 days. So that's roughly 2 weeks to save up enough for the pet and any other neat items the Hallow's End vendors might be selling this year.
One week for the Feline Familiar doesn't seem so bad, but add in the fact that the event is during the Blizzcon Weekend and some people may have their free time to collect Hallow's End currency cut short.
Personally, I think it's somewhat poor planning or just really bad RNG for the two events to overlap like this. Hopefully the revamped content and Blizzcon itself will more than make up for it. :P