If one is afraid of text, feel free to skip.
TL;DR. Artists are generally always wanting to do better than their last project. They always try to push push push. You give them extra resources they will take it, fill it and demand more. That's just how we are. Milk every drop.
Each company has different ways and pipelines for making assets. Figuring out a good pipeline to get a character from the concept phase to in game and working is crazy long.
I'll give you a basic pipeline from start to finish.
Concept Phase, pretty self explanatory. Batch of thumbs and designs till its whittled down to one core design.
Modeller gets that, talks to rigger/animator if there is any special needs this character might require.
Character modeller models a low poly base mesh. Passes this off to the rigger as a proxy. (if the rigger requests this for a special case)
Character Modeller then takes the base mesh to a high poly stage. This may require a re-topology stage during this (usually if they pre-planned their base mesh well or the mesh isn't very complex this isn't required). Where the they reconstruct a new base mesh with better poly flow to create a cleaner high poly.
Then character modeller gets the high checked off.
Character modeller then does a final re-topology of his high poly asset to a lower poly in game version. This is always LOD0 (highest polygon version) obviously. (this job can be palmed off to a specific artist who only handles retopologising, UV unwrapping and baking)
NB: This is the part where they will be doubling their polygon count for next gen machines.
Then comes the UV unwrapping of the low poly game mesh and packing of UV's
NB: This can take a little longer if you have more polygons to deal with. But its not so bad these days with the tools available.
Then you need your different bakes from your high poly source mesh. This can be quite a complex process if there are lots of fiddly overlapping objects on the character. Things need to be pulled apart and baked separately. Usually two maps will be baked. The normal Map and the Ambient Occlusion (AO)
You can get more like displacement, height and cavity maps baked if needed.
Then you go from there onto the texture phase. This can be handed over to a specific texture artist.
NB: This is where you may get higher texture budgets. Painting a higher res texture takes more time. Also if more maps are needed for shader tricks, then more time is spent making special case textures.
After that its setting up all the sharers for the character. Depending on your shader pipeline (something that is worked together between tech artist/FX artist and code)
Then once this has been done the rigger then gets the model.
NB: Shader complexity may increase for the artist to hook up. But this is all dependant on the specific shader pipeline of your engine.
The character modeller then makes the needed LOD's for that character.
The rigger or Tech character artist, sets up all the final rigs and controls. Usually sets up the collision and all the technical gizmos that are needed like Sockets and code controls.
NB: Next gen machines may allow more bones per chunk. So this may add more time in the rigging phase to set up.
Then this is passed over to the animator for animation. Blends, animations and all that.
Then the animations need to be hooked up to the game design and code. So we can say when SPACE BAR is pressed, character plays Jump animation. ect ect.
NB: With more bones per chunk, Animators may need to spend longer amounts of time animating (Yes there is mo-cap which has its own stupid crap to deal with. cleaning up keyframes.. bleh. Also it can't be used for lets say, a crazy squid monster of doom). Though I am not an animator, so I'm only talking from observation.)
Then the character is finally in the game.
Wow finally its in the game! Oh shit... what happens when, for example, you show this to the publisher and he decides you character needs to wear a cloak? Then you have to dismantle the pipeline back to the modelling phase to add the cloak. the rigger/character tech artist then needs to re bake weights and add the new bones and re rig the bastard. Then the animator needs to add all these cloak animations to his existing animations.
One fuckup or one change can bring the whole pipeline to its knees. The more complex it is the harder the pipeline crashes.
Do this times X number of characters in a game. It can add up.
Now environments that's a different story. You usually only need to do up to the collision part. (Unless you had to make a complex prop with animation of course) We need to make collision meshes (soooo boring) and try to find new ways of saving drawcalls while adding more to a scene. Environments of course need a variety of shaders for different materials and different shaders on the one object also means more drawcalls. For example a tree will need two shaders because wood behaves differently than leaves. Its a discipline quite different from characters but takes just as long because the basic process is the same with the baking and the source models.
NB: Artists are generally always wanting to do better than their last project. They always try to push push push. You give them extra resources they will take it, fill it and demand more. That's just how we are. Milk every drop. If a frame rate is shitty in a game its because the artists pushed too hard expecting code to keep up. 
For reference I have been both a character and environment artist for a next gen single player game utilising unreal engine 3 and also a next gen MMO. The pipeline I described was what I used when making characters for the single player unreal3 game.
