There is quite a bit of data online for external flash/speedlight durations, but I've never seen any data for the built-in flash. Has anyone ever tried measuring the duration of a pop-up flash at different power settings? I'm especially interested in Canon DSLRs.
Keep in mind that the flash duration varies according to the flash power, and it is not easy to directly control flash power for a built-in flash. Usually, at lowest power, the flash duration will be shortest, while at highest power, it will be longest. However, some studio strobes reverse this, with shorter flash duration at higher power than at partial power. (See Flash Duration of Speedlights: How long is the pop of a flash?)
I haven't found measurements of flash duration for built-in flashes, only small flash units. Actual Measured Flash Durations of Small Speedlight Strobes. This does however show the clear relationship of flash power to flash duration, and some typical flash durations for small flash units.