There does not seem to be widely published measured flash-duration data specifically for Canon DSLR pop-up flashes. In general, though, flash duration depends on output power: at lower power the burst is usually shorter, and at higher power it is usually longer. Built-in flashes also make direct manual control of flash power difficult, which makes measurement and comparison less straightforward than with external speedlights.

If you need a practical expectation, small built-in flashes should behave broadly like other small flash units, with duration varying noticeably by power level. One caveat: some studio strobes behave differently and can have shorter durations at full power than at partial power, so you should not assume every flash follows the same curve.

So the best answer is: specific Canon pop-up flash duration figures are not commonly available, but duration is power-dependent and likely comparable in trend to other small flashes.