So, if we're talking about visual reworks, it can take a while. Despite the character being already established, the in-game asset is completely different. This breaks down into these resources needed:
Concept
3D modelling
Texture
Rigging
Animation
VFX
So, that's pretty standard for any champion, original or rework. A new champion currently has 1 skin it releases with. A reworked champion can have a lot, take Ashe or TF for example. So, in the case of Ashe, let's say, that's five skins plus base. Despite two of her skins being essentially recolors, that's still 3 others that need new models, new textures plus custom rigging and animation support to flesh them out. Remember, no original rig remains. That's a really big deal if you know anything about animation. If you don't. trust me, it's a big deal. Depending on those skins, that might be several different sets of particle effects, again, depending on complexity.
If we're doing a severe overhaul like we did with Soraka, that's pretty much taking on a brand new champion because we've brought in the other disciplines such as creative to work on new VO and other story elements. Also, since the old Soraka was appropriately disposed of in a back alley never to be heard from again, that required a brand new concept of her which required the standard amount of scrutiny and sign-off.
Twisted Fate was another such situation. Pretty much a reboot in a lot of ways.
On top of all that, add to the fact that it is our mission in these cases to retain the iconicism of a character beloved by millions and increase it's fidelity and consistency across the board. That's a pretty hefty job. With an existing champion you have a huge open sandbox. With a rework you have a framework that you must preserve. It's like reconstructing a historic landmark.
Just the other day I joined the relaunch team. It's something I plan on talking about more extensively in a couple of weeks. We're currently a pretty small team with not a ton of resources compared to Champion and Skins. But we've got a huge responsibility. I volunteered to be the dedicated concept artist for the team because we were lacking such a resource and right now I feel it's the best use of my time. I want to be able to take what everyone knows and loves about the champions on our list and be able to bring them up to the standard we now have with our current champions, but also to take some of that consistency we apply to our reworks, apply that to current champions and project that to future champion development.
By reworking the past we therefore improve the future. When we make the present consistent, we improve the past. Sort of Orwellian now that I think about it...