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<blockquote>The flashing-light frequency shall be 90+/-30 times per minute.</blockquote>

I don't know what could be more clear.  This spec says that the light must flash 90 times per minute, &plusmn;30 times per minute.  In other words, the flashing rate must be from 60 to 120 flashes per minute.

It should be obvious that this is 1.0 to 2.0 Hz.

<blockquote>What will be the rise and fall time of the PWM coming from this IC</blockquote>

I only looked at that datasheet briefly because this is a silly thing to worry about.  The switching is implemented with digital logic and transistors, so is going to be "fast" compared to the switching rate.

Think about it.  They need the output switching transistor to be full on or full off most of the time to not dissipate too much power.  Even a ridiculously long rise and fall time for solid state electronics will still be instantaneous in terms of human perception, which is what ultimately matters for a turn signal of a motor vehicle.