Since the top and bottom values of the Avionics bonus & Flux bonus are inclusive, the probability are actually 1/71 and 11/91.
This gives a probability of 0.00170252282928339% of getting this item per mission.
This means that, to have 99% chance of getting this item at least once, you need to run a mission 270489 times.
If we estimate 10min per mission, that's a total of 45081.5 hours of missions, or about 15.4 years if you do mission 8h per day every day.
By the way, if you aim for a very specific reactor (max Avionics & Flux, and a specific bonus), the time required for a 99% chance of getting it is 1487700h, or about 509 years at 8h a day.
I love Maths.
Edit : I just discovered that each house give access to only a few modifiers, so the maths changes a bit. The probability from AnruKitakaze's post doesn't change (factoring one good bonus amongst the two bonuses possible instead of 3 out of 6 has the same result).
However, if you want a single specific reactor, the time required goes down to 495900h, or a bit less tha 170 years.
Maths is great.