Could you elaborate furthermore on: "I've read somewhere that the time it takes for the average VFR pilot to enter IMC and completely lose control of his/her aircraft is around 90 seconds."
From the CASA guide that Clearedtoreenter linked,
Spatial disorientation is the big danger. And it can happen a lot faster than you might think – just 178 seconds on average, about the length of a commercial on TV .
That estimate is based on studies in the 1990s by aviation researchers at the University of Illinois. They took 20 VFR pilots and got them to fly into IMC in specially programmed flight simulators.
All of the pilots in the study went into graveyard spirals that would have ended in uncontrolled flight into terrain or roll- ercoaster-like oscillations that became so intense that they would have resulted in structural failure of the aircraft.
In repeated tests on the simulator the result was the same – all pilots lost control of the aircraft. The outcome differed only in the time required before control was lost which ranged from just 20 seconds to 480 seconds.
It is counter intuitive but very real. Part of ATC refresher training for this scenario involves a tape replay of such an incident. One of the scariest parts was the pilot saying "I think I'm upside down".