If you are going to use Lightroom then you need to use Lightroom to do your cataloging. The catalog is the only file Lightroom opens. The catalog is a database, and your images are displayed in an image object. It will be necessary to import all of your images to build your Lightroom catalog. It's difficult to estimate how long that will take.
I am upgrading from CS6 to CC this week. Since Photoshop 4 I have kept all of my image files well organized file structure on one disk. The first drive was 100mb. Currently I have about 2Tb of photos on a 6Tb drive. I have always used Bridge because when LR first came out I heard horror stories of what LR cataloging was at that time.
That's funny, I have heard the horror stories about doing this in Bridge. But if we are dealing in facts rather than horror stories ...
If I created a single catalog from the 2Tb+ images on the local 6Tb drive how long will it take LR to catalog it and will LR have to catalog each time LR is opened?
Each time LR is opened, there is maybe 10-15 seconds depending on the speed of your computer. That's it. Lightroom doesn't perform any "cataloging" operations at that time. No one can guess how long it will take to import the photos.
What would be the advantages of using LR to catalog vs Bridge. Can you use LR without the catalog feature?
In my opinion, using LR without taking advantage of the Library Module features is pointless, you still have to import the photos. You can't avoid the import step. So ... all the keywords and captions you have applied in Bridge can be imported into LR and used from LR. The advantages of using LR are primarily that Lightroom is a database and so there are many powerful searching advantages that you have in database that are not present, or very slow, in Bridge.
It also doesn't make sense, in my mind, to use Bridge for organizing/searching and Lightroom for editing, I just can't see a reason to split this up.
I have kept all of my image files well organized file structure on one disk
This is fine, but the advice I have is to transition to metadata organizing in Lightroom, rather than folder organizing. Again, there are many more features that Lightroom provides that makes metadata organizing more powerful than folder organizing, plus beginners seems to have all sorts of troubles continuing to use their folder organizing in Lightroom.
A Catalog in LR is like any other type of Catalog. It is a Listing of what is on display, for sale or auction, or data.
With LR it is a listing of your images files.
To import, Catalog, 2TBs of images files into the LR Catalog file, IE a Database file, could take some time. It all depends on exactly how many files are contained in that 2TBs.
But once that is completed LR won't take much time to open. The actual size of the catalog file and the total number of images Cataloged in the LR Catalog file doesn't have any affect on the opening time of LR. That is more dependent on the speed of your CPU and storage drive the catalog file is stored on. So Even if your images are on a rotating, old style, Hard Disk Drive place the catalog on the SSD you use for the OS and programs (If you can and if you have a SSD in your system).
Bridge is a File Manager. LR is a Database system. Bridge only displays images from the folder you have selected. LR can do that also but it can also display all the images imported, all the images from any drive that you have imported images from and selected or all the images from a Top Level folder including all the subfolders under that Top Level folder. You can also use the Filter system along with Smart Collections to display only certain images or types of images.
You don't have to change the way you store your images. You can still place them in folders and subfolders by date or place or client or whatever way you want. I still use a folder system based on Years then subfolders for dates, month and day, in the years top level folder.
No you can't use LR without the catalog feature. That would be Bridge and ACR.