Just finished trying out sub-surface mining at a single Tritium Hot spot. MUCH better yields, but also much more difficult to do at my level (stupid rotating asteroids!).
50 minutes for 134 Tritium (128 in cargo, 6 in hopper). I found one sub-surface launcher didn't contain enough ammo for me, so I needed to synthesize ammo twice (which means I will need to look at getting some of the required materials like Tungsten planetside).
At times my rate was higher, but it depends on the rotation speed of the asteroid and the sub-surface mini-game layout. Overall still I feel the extraction rate is still too low; and SenseiMatty's suggestion of 250 tonnes/hour sounds like a fair target rate. Perhaps I can get close to these rates with a lot of practice of sub-surface mining, but if another player dislikes mining after giving it a good go, well it probably would be good to have another option.
I do like the suggestion of passively fuel-scooping Tritium around gas-giants that came up in beta as well. I think 48 hours is a bit high, although fixing it at my current mining rate which is about 6-8 hours per 1000t of Tritium may make it too easy; then again mining in combination with the passive refueling would be like having a second commander helping you to refuel the tank.