I have read a lot about transmission fluids, what good ATF looks like, and when to change it. But, I still have trouble defining the deterioration level of transmission fluid I extracted from my 2008 Camry (See attached image). It looks quite muddy but it has no burnt smell. What you think, how deteriorated is this ATF? Should I have changed it before it started to look like this?
The colour is too dark, this fluid should be changed. As mentioned in the comments, automatic transmission fluid lives in 2 places: in the drain pan and in the torque converter.
Some cars have a drain plug for the fluid in the pan (that's as simple as a motor oil change), others don't and you need to remove the pan and change the gasket (that's a messy job).
To get to the fluid in the torque converter is more involved and usually can't be done at home. You don't need to go to the dealership, any mechanic shop can do this. Find one that you trust.
If the automatic transmission fluid was "dirty red," you could have just drained the fluid from the pan and replaced it. But this fluid is too dark, I would have the entire system drained (torque converter and pan) (it's simpler), or drain the pan and refill multiple times.
For this last option, you'll need lots of fluid and you'll need to drive the car a little bit between flushes to get the fluid to mix. Basically, this will dilute out the old fluid, you'll probably need 10 to 12 flushes to get the ATF to "look like new." The amount of ATF needed for this will cost the same if not more than getting a shop to flush the entire system (1st option).