Timing mental arithmetic training for specific event
Firstly, I think you’re thinking too much tbh. But still, I would totally answer this question- I think both are good approaches. But at different period of time. Suppose it’s after a month. So for the first 20 days you work on different time period. It’s better you work longer- what I mean is you start by giving one sample test in one hour then take a 15 minutes break. Then again you do it for 3-4 hours. Take a good nap for 1-2 hours. Then again do it. 
In the last 10 days. You practise hard at 12PM  for sure. But you also practise in the remaining time. Because you need the brain to work. Simple example- if a car is switched off for 30 days it will take time to start. But if it switched off 39 minutes back it will start asap. Think about it. 

I hope it helps.