I think that coding softwares or scripts can be a very long process. There are so many different files you need to code. So how long does it actually take to code something? I know it varies with different things. Forum themes? Forum softwares? Simple script with admin ACP? Forum plugins?
It depends on a couple of factors really, mainly the size of the project, but also the skill and amount of programmers (though the time to create a project doesn't linearly scale with the amount of programmers, there's a point where you just delay things by adding more).
For example, a simple shell script to automate a simple task might take 10-20 minutes and require about 20 SLOC (Source Lines of Code), whereas forum software like MyBB is probably on the order of 50,000 SLOC or more (and for that all the graphics would need to be made/found). Keep in mind that MyBB's been in development for about 13 years now, so there's a huge variation. One final example is the Linux kernel, which had its first release 24 years ago, and now has 16,000,000 SLOC.
Really, it's difficult to say for certain how long something will take, because there are far too many variables to account for.
Yeah, in terms of the length of projects, it really depends. It is like asking how long it takes to build a building. You can build a shack in a day. A skyscraper could take a year.
At least with the stuff I do for FP, I know that phpBB Extensions generally take up a few thousand SLOCs each. But those are scaled kind of weird given how much overhead there is in the dependency injection process.
It really depends, like the others have said. Even the same exact project could take different lengths of time from person to person. And you've always got to keep in mind that you need time for peer review, bug fixes, offering support, etc. 🙂