Is there a limit on how hot you can superheat water at 1 atm? Is there a limit on how hot you can superheat water at 1 atm in a perfect container? What about in a microwave?
 A: Amazingly someone has written a paper on this very subject (some people have far too much free time) and you can find it here. I wish I could claim this was my encyclopaedic knowledge of Physics at work, but it was just some lucky Googling.
Anyhow, the theoretical limit for superheating of water is (astonishingly) about 600K, but in real life you wouldn't get anywhere near that.
A: the superheat characteristics of water are well-documented in the (engineering) boiling heat transfer literature. Regarding your specific question, the maximum temperature to which water can be heated at atmospheric pressure is called the thermodynamic limit of superheat; here is a first-order derivation of it:
an undisturbed volume element of water in the absence of nucleation seed sites can be heated above its "boiling temperature" for a brief period before the random thermal motion of its molecules produces a transient void of sufficient size to function as a nucleation seed which triggers the vaporization kinetics at that temperature i.e., the water boils. 
The higher the temperature of the water is raised, the smaller the requisite nucleation void volume becomes and hence the amount of time required for statistical fluctuations to furnish a transient void which will serve as a nucleation site at that temperature becomes shorter. 
As the temperature of the water is raised higher and higher, this initiatory time lag asymptotically approaches zero. the temperature at which this occurs is defined as the thermodynamic limit of superheat which for water is 340C or thereabouts, depending on how many of the second-order details of the process you wish to include in your model. 
