What is the estimate for total number of flight hours in the world?
In 2018 there were about 37.8 million commercial flights. If we multiply this number with the overall average flight duration, we get the answer. Unfortunately I could not find a result for this directly.

We can however calculate it from the data provided by the MIT Airline Data Project. The data says there were 3,954 aircraft available on average in 2018 that accumulated a combined 13,398,545 flight hours with 3.82 departures per aircraft per day on average. This means the average flight duration was

$$ d = \frac{13\,398\,545 \, \mathrm{h}}{3954 \times 365 \times 3.82} \approx 2.43 \, \mathrm{h} $$

This is just based on various US carriers, so the world average might be somewhat different, but since US carriers fly a wide variety of flights from short domestic routes to ultra-long haul international routes, I assume this number to be close to the true answer.

The resulting total number of flight hours is then

$$ 37.8 \cdot 10^{6} \times d \approx \mathbf{91.8 \cdot 10^6 \, h} $$

This does not yet include GA traffic or military flights.