Up to 24 hours
Generally it should only take around 4 - 12 hours, depending the account's data size.
Amazon's API is a very complex data source though, so there can be various special cases where Sellics needs to double check specific data sets. The whole process may therefore take up to 24 hours - in some rare cases also around 48 hours.
Once your products start appearing, you can nontheless are already start to optimize them!