Please provide Maximum Junction temperature of ADG508. The value is not available in datasheet. Only Thermal resistance is provided.
You can see from the power dissipation in the abs max ratings section that the power derates to 0W at approximately 153C. So it can be assumed that 150C is the max junction temperature.
i.e. 470mW/(6mW/C) = 78.3C + 75C = 153.3C