It varies depending on the temperature. With hot water I soak it for around 20 minutes, but for cold it takes much longer (at least an hour Id say). I normally put it on overnight so not sure of the exact time.
An hour is fine but I was told at least 6 hours by simple systems although the instructions say 10 to 20 minutes. If I soak it before I go to work it will sit there for at least 12 hours before I feed it
I found it did soak quickly and I was feeding 4 lots in each 24 hours, so the lunchtime feed was soaked at 7am and fed at 1pm, next lot soaked at 1 and fed at 5 and then another lot to be fed at 10pm. The morning feed would have soaked overnight. This was in cool weather though, didn't use the product in warmer months. In warm weather it might not keep too well if soaked for a long period.
The colder weather means its taking longer . I have started taking it home and soaking in hot water . The monring one is done round 6pm with warmish water but left overnight
It's worth buying an insulated cool box to soak it in. It'll stop it freezing over night and if you take a travel mug of boiling water to add to your mix it'll keep the whole thing warmer for longer.