Kobold Fight Club is a great resource for this kind of thing. You can pick how many characters of what levels you have and what monsters you want to put them up against and it will tell you the experience and difficulty of the encounter your looking at by the encounter building guidelines in the DMG.

In this case, fighting an Adult White Dragon would be "deadly" for any four-man party 9th level or below, "hard" for a party of level 10 or 11, "medium" for a party of level 12, 13 or 14 and "easy" for a party above that. I've found that the parties I've DMed usually pull through fights a little above their weight class so I'd probably aim for level 9 or 10.