hmmm, god created the world in 6 days. but by reading most of the previous posts you can gather that it takes mere humans a fair while longer. I, myself have begun creating a spanish conquest mod. i spent 6 weeks researching all facters involved... Native cultures, arcitecture, navel vessels, soldiers, geological locations etc. Then i went through all research data printed it all out and high-lighted every bit of important data (some times whole pages are high-lighted). After that i then spent 2 weeks downloading files from this forum/community which i though i'd use, you always download more than you need. but anway, thats 2 months before i even wrote down on a piece of paper what i wanted/needed/intended to use... hundreads of pages of writing all organised in a file. Faqs and tutorials printed out and learned some times off by heart. that took up some of my time.
I started modding a year or more ago with civ 3 vanilla un-patched without the use of any tutorials what so ever as i didnt have the internet. trial and error takes up a fair bit of time.
I find that writing down what needs to be done quickly on a piece of paper for each task. Say, to add a leaderhead... i make a checklist of what i need to do. eventually you learn off by heart. when you know what your doing you waste less time but you still learn to test it anyway just to make sure you've done it right.
By the time you start editing rules and using the editor properly you've forgotten things like making sure newly added units are only available to the dedicated civ, it may sound stupid but these mistakes do happen.
editing general civ files such as pediaicons & civilopedia may seem like the hardest thing to do. but really it isnt, some times it may take more time than not to write a civilopedia entry from skratch, but most of the time its copy and paste. other things like map editing may take phenominal amounts of time to do/complete. but when all thats done, when your mod is finished. (im not sure about all the other modders but,) i play what i call a debugging game all the way through (includding all functions such as space race and so on) and list ALL malfunctions. once i finish the game and correct all the problems a play another full game to see if i've missed anything and to see if my corrections worked!
As i've never posted a mod/scenario before i havent experienced this personally, but the first few posts on other mods have been fantastic... and then all of a sudden people start posting problems which may dis-hearten you and you may even persuade your self to quite. DONT, the fact that youve manage to reach that stage in its developement says many positive things about you! user feed-back is the best thing that can happen to you at this stage as you can correct every mistake descoved by many many people and you eventually improve upon not only your mod but also your knowledge of the modding process and also that of the civfanatic forum and its other members.
i person have found that during the modding process, coming to the civ fanatic forum is like shore-leave. a coffee break during the D-Day landing

.
The most fantastic thing is that if you need help, if you ask for it... you'll get it. Personally, nothing beats being able to help others after you've been helped your-self.
My first mod was simple. I added in a ship of the line... i always hated how civ never really had a proper ship of the line shipped with it. that only took 30 minutes for me to do withoug any knowledge what so ever of civ 3 series editing. creater walk-throughs are a god send!
anyway, i've rabbled enough. but i have one more piece of info, take you time... dont rush it.