See my last edit
So if you are a completionist or wish to get all of the transmogs sets, I don’t see it being feasible.
For this going to ignore dailies and just use the data from rare spawns.
Each rare drops around ~30-50 stones (sake for argument going to use 30).
You turn in 25 stones for 3 Commendations.
It takes you 135 Commendations to compete just 1 set.
You have a total of 8 sets to get (4 armor types & Horde/Alliance versions).
Lets assume you are doing this event non-stop without large breaks and getting a rare every 20 minutes.
Single Set Goal: 135 Commendations.
You need to complete 45 stone turn in quests, each one gives you 3 Commendations.
Each turn in quest needs 25 stones.
Lets say you get 30 stones per boss.
You will need 1,125 stones to complete a set.
That translates to average around 38 bosses to kill!
Now take to the rotation times into the mix.
Each one takes 20 minutes so that will be a total of 760 minutes or around 13 hours just to complete a set!
If you want to get all 8 sets then you will need:
9,000 stones
Around 300 bosses killed
104 hours or around around 4 days (this if you do every single boss without a break)
Lets say you only have 4 hours in a day to play when you get home from work, that will take you 26 days to complete every single transmog for this event!
You only have 14 days for this event.
Edit: Made the mistake of the amount of items for a set, its not 150 commendations its 135
Edit2: I’m sure things can pick up after week 2 unlocks for dailies https://www.wowhead.com/guides/shadowlands-deaths-rising-prelaunch-event-scourge-invasions#pre-patch-dailies
Edit3: I can try to add the dailies and what you get on the intro quests but would take longer.
Edit4:
So looks like you only get 23 commendations on normal quests for the 2 week period. Only enough for 1 piece.
Week 1 normal quests is 13 commendations.
Week 2 normal quests is 23 commendations (includes week 1).
Horde seems to have the only city daily as I can’t find the SW one, that is 5 commendations. That quest is an unknown.
Week 1 Dailies:
12 Commendations per day
84 Commendations for Week 1 Dailies
97 total Commendations for Week 1 Dailies + Quests
Week 2 Dailies:
9 additional commendations per day
147 Commendations for Week 1 and 2 Dailies
170 Total Commendations for Week 1 and 2 Dailies and Quests.
That is 11 pieces of gear with some left over.
Ok, I now see how dailies will be the factor. I was only using the boss farming.
If someone can double check my work.