I know that many artists varnish oil paintings early. I always try to wait as long as possible. Of course it depends also on how thickly you paint. If you can easily dent the paint film with a fingernail, it’s too soon to varnish. Different problems can result depending on the mediums you used and the type of varnish. I’ve had paintings that never dried. I’ve got two in my studio that I varnished too soon. That was about three or four years ago and they are still tacky!