A typical starting point is to assume a Vf deviation of ±10% to ±15%, but you typically buy LEDs binned by their measured forward voltage.
As an example, we’ll use the OSRAM 3W OSCONIQ® P 3737 as an example: The datasheet lists the forward voltage range as 2.60V to 3.10V with a nominal of 2.75V – in other words, -5.5%, +12.7%. Since you buy LEDs binned by forward voltage, the actual deviation is much lower within a single part number. For example, the voltage range in the L2 bin is ±1.7%
Note that this does not take into account the forward voltage change due to the Vf temperature coefficient (the temperature coefficient will be very slightly different for each individual LED, but this doesn’t matter too much in practice – so you can mostly assume that the forward voltage temperature coefficient is constant).