Hi, Martyn!
We had a somewhat unique case, but I think you might still benefit from hearing how we worked through it. By the time R12 came along, our model had grown to epic proportions. Not in terms of usefulness or functionality, but in terms of size, complexity, and its ability to make us bang our heads on our desks. Much of our model had been built using technical, hands-on-keyboard help from Apptio, as well as TBMAs who had moved on to other teams. Because of this, no one on the TBM team fully understood how the model worked. We decided that when we upgraded to R12, we'd start over from scratch.
You have two full-time TBMAs, which is awesome. We had none. There are three of us on the TBM team, but none of us are full-time TBM. We have the finance person, the enterprise architect, and the developer (me). As the developer, I was chosen to build the new model...myself. We did the upgrade to R12, but lots of things in our R11 model immediately broke. In the meantime, I spent some time planning what the new model should look like, and then beginning the actual building process. Overall, it took me about four months to get the model up to a point where it was providing the same information as the old one did in R11. The difference, though, is that the R12 model is much less complicated, and I fully understand how costs, assets, and resources flow through it. The bad part is that I'm the only one on the team who does, since I'm the one who built it. It gave the rest of the TBM team an excuse to just send everyone my way whenever anything goes wrong or anyone has a question. But, the model works now, and I can (relatively) easily fix any bugs that arise.
tl;dr
If you are building a model that takes into account labour, fixed assets, vendors, etc., goes through ITRT, and up to applications, projects, and services, then it'll probably take between 600–800 hrs. to plan, design, build, and test the new model in R12.