What is the right time? that is different at every wedding and depends on many factors- the weather, the atmosphere, how long since the meal finished, how big the meal was, have the evening guests arrived yet and had time to have a drink or two and a few others as well. If the venue has a nice terrace/veranda etc and the weather is very warm they invariably will stand outside for a while chatting and this will often delay many people being ready to dance.
However one thing is for certain it is'nt when the venue tells you it should be nor is it when the photographer tells you when it should be.
It is when you say to yourselves "we are ready" that may be after you have had a sit dow to gather yourselves or it may be after the buffet or whenever you want to as it is your day. do not bend to pressure.
Have you ever been to a wedding where the first dance has been done very early in the proceedings and then afterwards the dance floor is empty for a long period?
I like to think that the first dance kick starts the night if it is at the right time so take control , you decide.