Does the ditching switch allow an A320 to float indefinitely?
The answer is no, not totally, but it would really slow things down. I don't think anybody knows the precise answer because only flat water ditchings seem to result in the airplane stopping in the water in one piece (such as 1549 and a similar one in Malaysia) and flat water incidents (like Malaysia and some airport overruns) are usually in shallows where the ditching valve is moot. 

But assuming an A320 was able to ditch on open ocean swells without breaking somewhere, certainly it would float for a much longer time, but not forever. Pressure hulls are never absolutely fluid tight; door/hatch seals leak a little bit, and there may be tiny leaks through various entry points below the water line like bleed air shutoff valves, fay-sealed skin laps, rivets, control cable pressure bulkhead fairleads, etc (the older the airframe the leakier).

Wing dry bays may be sealed off with a tape made for the purpose but they are not usually totally water tight.  Water can migrate into the fuel tanks through the vents.  Full fuel or nearly full fuel would be a bonus, fuel being buoyant.

You certainly would have a lot more time to get onto the rafts (perhaps Airbus has estimated the theoretical sink time by calculation), but it would sink eventually (it would take quite a few hours, maybe even a day or two), and you aren't going to be staying on board any longer than necessary regardless.

(Now, if you ditch in a composite foam sandwich sailplane, well you've got yourself an unsinkable boat right there, and if you took a paddle along, you could actually go places.  If I'm in a glider that gets low over rough forested terrain, and there are lakes around, I'm putting it in the water and paddling it to shore with whatever I have at hand).