The drying time of varnish can vary depending on the type of varnish, the humidity and temperature of the environment, and the thickness of the coat. Generally, it is recommended to wait at least 24 hours before applying a second coat of varnish. This is because varnishes are resoluble, and the more time you allow between coats, the less prone a varnish layer is to reactivate. More drying time also facilitates easier secondary application.
It's important to wait for the varnish to dry before applying the second coat. The varnish should be dry and not tacky before the next coat is applied.
It's also important to keep in mind that the drying time may be affected by the humidity and temperature of the environment. High humidity or low temperature can slow the drying process, while low humidity or high temperature can speed it up. It's recommended to consult the varnish manufacturer's instructions, as they may have specific recommendations regarding the drying time before applying a second coat.