This will be a bit of a non answer as the question somewhat leads down the wrong path. The short summary is that with efficient training and a few seasons under your belt, you can be a successful cat 3 racer on 5-7 hours per week. Once you get above that level, you are entering into a completely different game and while more time will be needed, it will be highly dependent on your personal goals.
Anecdotally regarding time trials, I'm a Cat 3 racer with a couple of wins and I have never done a 40km/h time trial for a flat 15km race, let alone a 40km race. I race with a lot of guys who could do a 40km/h TT without aero equipment.
Time trial performance is at best a minor indicator of success in the amateur levels. It indicates potential, but little else. It really only helps in time trials, races with substantial climbing and some very specific situations in mass start road races.
From a fitness standpoint, racing success is more about repeated high speed surges that you need to recover from. From a mental standpoint, you need to be able to conserve your energy as much as possible in a race until it's time to use all that energy. Knowing when that time comes is also a bit of a challenge.