This is not a problem. But I don't know how long it is enough.

"But I don't know how long it is enough." Good question! I don't know either. To give an idea: This: http://www.ngspice.com/index.php?public_circuit=5djLxc (ignore the fact that it doesn't work properly yet!) is clocked at 250kHz. To properly test a SMPS simulation you need to apply load current pulses and see the transient (time domain) response to check that the circuit is stable and so on. There are times when you may need to see all of the startup time as well as the later steady state times. For example to see what happens if a load is pulsed on an off during the startup time and continues to do that into the steady state? In some designs it may be 10's of ms before the circuit has reached a steady state so simulations may need to run for a long time of many. many clock cycles. In an ideal world, there should be no upper limit on simulation processor time and datafile size. I regularly run sims that take many minutes to run and generate 5Gb simulation outputs. A guess is that a 250kHz SMPS should be able to run for about 200ms with a mix of maybe 6 voltages, currents and powers being probed. Maybe a shorter time for an SMPS running at a higher clock frequency but the run time is very dependent on load conditions. Same with Class D amplifiers: you need to be able to run signals through them and look at stuff like power supply noise (hum, transients) rejection. The models for some of half and full bridge drivers I've built for - and even the discrete UM10155 - class D amplifiers are quite complex so again long run times can generate a lot of data. Another one would be the 4060: to see the MSB just go through one cycle requires 16384 cycles so a newbie who wanted to use this to generate a clock (maybe divided down from the mains) for something else would need to run a very long sim ... OK, that's a bad way to use a simulator: a better way would be to replace the 4060 with a PULSE source at the required low frequency but that is not how most newbies think! They need to be educated to think about using tricks like that. :)