I only tend to change mine occasionally. I have gone whole expansions using the same thing.
At least once a week, I guess, but I have a bunch of mogs saved and just rotate between those.
people change mogs?
inb4 the forum changes my mog again!
Probably 4-5 times every session. Its an expensive trade and its where my goldsink is
Every 2-3 months, I like to collect mogs, but I’m not creative enough and mostly stick to sets.
I couldn’t give a fixed amount/time. SonI just say it as I do.
Full transmog maybe once or twice per week. But I do little changes and change certain pieces sometimes multiple times per day.
So the best straight answer would be “often, very often”
Depends, I tend to stick to my “main” mog unless I need a more appropriate disguise outfit for something 
In cata i dont use Mog, i like to wear tue set of the current raid. (Thankfully i like all hunter tier sets a lot)
Aside from the xmas mace I had equipped and now removed, I refuse to pay the silly amounts that mogs cost. It is 1000g for a full mog change for me on a few characters now. Blizz need to remember that mogs are good advertisement for the game. The more that avoid paying for mogs the better imho. They should a free item to use as you wish. It was like that in remix, so why not in retail?
i change it to the Elite pvp set each season,
get bored of it and then switch back to my favourite sets
If we exclude minor changes i am using same mog since BFA.
I tend to swap around, but most of my characters now have a “One True Mog” where I will always eventually return to.
Pretty much never, unless I come across a skin that makes me go wow, but that hasn’t happened in a while
… rarely - once I get the right fit for a character, that is it - there may be the odd item that I switch I.e. my undead frost mage changed her back piece recently as it suited the outfit better.
Therefore, I have no idea why I still keep collecting transmogs 
I don’t. I have a basic mog which most of my shadow priests use though I do get fed up with having to shell out gold when I get an upgrade and have to pay to put the same mog back that was on the old one.
Without sounding all to spiteful---- I changed my mog once since I’ve race changed, and by that i mean i visited the barber and changed from black to black & purple with purple armor instead of just black
tis still such a disgrace that dracthyr can’t wear a full armor set its laughable
I have used the same mog for years. The only reason this char sometimes look different is that I tend to only mog after I’ve gotten most/all of what I can get
Which reminds me it’s about time to visit the tmog guy
Depends on the character.
I have mogs for every spec on my DK but I pretty much use the same mogs as I like them a lot.
On my Warrior I keep only one set (t3, how original).
On my priest I use the same mog as well, I love the mog of my SP.
On my rogue I change mogs more because you can combine different leather pieces together and will look decent, though I always gravitate towards a similar style when comes to mogs.
In simple words, more often than not compare to most I guess.
I usually change my look every few months. I’m on my second mog in S1 I’ll hopefully get a Hero level helm soon from Delve vault slots so that I can switch up the tint on the Pally tier set.