Bodypart: 1/2 Days Bodypart pack: 4/26 Days Stock configuration: 5 Minutes/4 Hours Car (scratch): 5 Months/2 Years Car: 2 Months/7 Months Depends on the mod.
porting a body part for me doesn't take too long, for example, the entire hillclimb caravan mod was completed in ~3 hours. I simply used mesh offsets and manually moved all of the nodes from the parts on the hillclimb pessima. Mesh offsets, if you've never heard of them, are explained quite well in part 4 of @wrinkle345 's post, https://www.beamng.com/threads/tutorial-porting-engines-and-other-parts-inbetween-vehicles.28730/
As a newbie, I needed over a year to develop my first mod: lack of initial knowledge, limited time to work on it and overall complexity of the vehicle in question determined quite a long development time, even if about 85% of the graphic assets were outsourced.
For Level building; it has everything to do with the complexity of the design. If you're just planning on a small simple map, using stock assets and textures, it could only take a week or soo. On the other hand a large real world level with lots of detail and good accuracy could take 100's of hours. The average scratch built race track (ie: for a sim like rFactor) is around 2 months for a professional to finish (about 300 hours). 1) researching the location and acquiring photos, maps and elevation data; 2) Modeling the base geometry (terrain and road layouts) plus walls curbs buildings, textures etc. 3)Coding any AI, or special scripts. For this game, producing a professional quality level its simplified in some ways, since the terrain is a height map grid, with vertex painted materials, so you don't have to do any actual 3D modeling or UV mapping manually. I have released 3 levels for this game. My grid map style half pipe map took only a week or soo. My real world ones are the most intensive to complete, because of the development phase, finding the newest most high fidelity resources. Old blurry pictures and sketchy maps make for poor race venues. Ive been plotting away at Road Atlanta for...10 years on and off.
Everyone's time frame is different, and everyone has different circumstances. An average time is pointless because there are so many different circumstances for people, that at the end of the day you've just created a value that is meaningless. It can take as little as a couple weeks, to as many as several years, for a mod to be made. A mod is done when its done, and its ready when its ready. Patience is key, and you must be understanding when someone cancels a project for whatever reason, or the project gets put on hold. Simply put if you're not making it or involved in its development in any way shape or form, you have no say in what happens to the mod or when it should be released.
Assuming you already know how to use the tools well (like Blender, the Beam map editor, etc) and you work on a mod 1 hr a day every day, I would venture to say that the average Vehicle mod would take 2-3 months Map mod would take 1-2 months (shorter for basic or simple maps, longer for large or detailed maps) For ui apps and configurations it varies widely
Levels usually take me a month or more of on-and-off work. That being said, I also tend to go back to levels I've made and do more work on them. I mean Conquistador Gorge was dead for a few months but then I came back and completely overhauled it in a few weeks. Of course, I have a ton of models w/ materials pre-mapped out so that makes things easier. However, there are also numerous ways to go about making the level which all vary in the time it takes. I just rip terrain information of an interesting location from terrain.party, and import it to L3DT. I do a little cleaning up in L3DT and then import the heightmap into the game. From there it is a matter of laying out textures, objects, lights, sounds, and etc. Configs and skins though take far less time and, to be honest, I'm actually finding them substantially more enjoyable to make compared to maps (but I might just be burnt out from making maps). Configs with custom parts and skins usually don't take more than a few hours - if that - at most. Most of that time is spent actually making the skin (importing public domain textures, painting, text editing, and etc.) I've been reading blender tutorials so that I might actually get into making true custom parts (not just jbeam re-purposing.) But everyone works at a different pace - and that isn't a bad thing. Some people work slow and finish their stuff when they darn well please while others go full hummingbird-mode and make their stuff extremely quickly. It all depends on a weird mixture of the talent, prior knowledge, and passion for the project.