This depends on the model, as the video timing is driven by which Spectrum's video timing is currently used. Which one depends on either the personality chosen, or what kind of tape load you do from the NextZXOS browser.
With VGA or RGB 50 Hz, 1T = 1/3.5MHz 48K = 224T * 312 = 69888T per frame = 50.08Hz 128K = 228T * 311 = 70908T per frame = 49.36Hz Pentagon = 224T * 320 = 71680T per frame = 48.83Hz
This is at VGA 0, the perfect timings where the system clock is 28MHz. As you move up through VGA 1 to VGA 6, the system clock is increased according to nextreg 0x11. Then the refresh rate has to be modified.
For example, at VGA 5 the system clock is 32MHz. The spectrum runs at 32/28*3.5MHz = 4MHz in real time, and the refresh rate is 32/28*50.08 = 57.23Hz.
The video mode chosen should always be VGA 0 or RGB at 50Hz. This option of increasing steps VGA 1-6 are for monitors that cannot sync to 50Hz only. All relative timing in the machine is kept the same but the real time speed is higher so programs run faster and sound is higher pitch.
The 60Hz VGA timing should only be used if the monitor can't do anything under 60Hz, or if the higher 50Hz settings are too annoying due to high speed. But all the relative timing relationships are broken, and programs depending on timing will not display properly, just like with HDMI.
If the display can only do 60Hz VGA or the higher 50Hz VGA settings is too annoying, We would honestly recommend looking for another, more capable monitor.
The pixel resolution of the VGA signal is 704 x 568.