I've been wanting to get into modding, my idea for a first mod is to build a home. I want to make the home a small hidden underground fortress used by the dragonborns of old. I was wondering how big of an undertaking this would be for a first time modder and if I should maybe start with something more basic? I've been watching the tutorial videos but I'm curious as to how long it would take to build something like this and have it be high quality. Thanks guys.

depends on how big of a place you want to make. A simple 1-3 room house can take a couple of days

Yep depends entirely on how complicated you want it to be both in size and functionality, For the most part it is just drag and drop for a simple home. I always recommend a house mod as a first project because you have the possibility to cover pretty much everything the Creation Kit can do while still keeping the size somewhat small.