There are no linear power supplies in any computer. They would be far to large and heavy to accommodate the required output.
Time spans for damage cannot be calculated due to the enormous number of variables involved. Too many of which are not static.
Switching power supplies are regulated by the output demand. They are a closed loop system. This is why they need a load to provide power. Without sufficient load, the supply will simply not provide any power. Here is a grossly simplified explanation; Switching supplies work by increasing the frequency of the power as the load increases. If the load drops too low and causes the frequency to be below the input frequency, then all manner of noise is introduced into the output.
Without knowing specifics of the supply design, you can only make some gross assumptions about the operational range of the supply where clean power is produced. Generally speaking it is between 60 and 75 percent. You might get away with 50 to 75 percent, in a good supply, with an active PFC and decent capacitors.
One of the main problems with dirty power is it usually is mistaken for other problems with a computer. Stability issues, memory errors, hard drive problems and a host of other issues can all be traced back to dirty power as a potential cause.
As the voltage level requirements drop for components, the higher potential for dirty power to cause more problems. Hence, what you could get away with five years ago, would fail miserably today.
