What is drying time of dewaxed shellac before I apply water based top coat?
If your shellac is fresh the drying time should be measured in minutes, literally as soon as the alcohol has evaporated shellac is considered touch-dry and it should no longer be tacky*. A very thin coat applied to bare wood can dry in as little as 1-2 minutes (!) in a warm room, but any thicker applications should be expected to take 10-20 minutes.
Note that after initially drying shellac isn't quite hard and you may want to wait 15 minutes to an hour before overcoating it, depending on your local drying conditions and how many coats you've applied.
*If it remains tacky for an extended period of time, hours or longer, it indicates the shellac has aged and the common advice is to discard it. As much as possible shellac should be made up fresh in small batches (or bought in small containers, paying attention to the production date) so that it gets used up and isn't stored for months and months.