Sometimes no time at all. But it does depend on the current health status of the horse and how well it has been managed from birth. The fastest transition I have seen was the length of time it took to take the shoes off the horse's feet (the horse has never faltered since.) The longest is two years and still going (this one will always need boots because the diet is not under control).
The hardest part of transition though is often the changes that need to be made by the main carer. If they are unable to move from traditional thinking and horse care to adoption of the whole DEET approach then the hooves may never transition successfully.
The above photos are 'pairs', they are of a particularly challenging case. The first in each pair is dated 27 04 10 and the second is 16 07 10. This foot was regarded as slow growing when shod.
In under three months at least a third of the hoof has been replaced, which is about average.
When deshod the soles were paper thin with shallow collateral grooves, no heels, lots of thrush, thin splitting walls, flares, white line separation, very little water line and a bull nose. Plus (I believe in an attempt to dress away the bull nose) the front of the hoof wall had been heavily rasped well above the bottom third of the hoof wall.
In the later photos, the sole is still thin, but not quite as bad, the collateral grooves are getting deeper, the water line is developing, as is a heel buttress and the white line separation is not quite so extensive. But this hoof is going to be a long time in recovery.
However this doesn't mean you have to stop riding your horse, but for it's health and protection hoof boots should be fitted for exercise until the sole is much more robust.