Is there a convention or standard of the highest bits in a sequence logo?
This is explained in the original paper [1] describing the logos (emphasis mine):


  The importance of a particular position in a binding site is more clearly
  and consistently given by the information required to describe the pattern
  there [7, 8]. To choose one symbol or state from two equally likely possibilities
  requires one "bit" of information. For example, to communicate the result of
  a coin-ip to someone requires 1 bit of information because only one yes-no
  question needs to be answered: "Is it heads?". If a position in a binding
  site always contains one base (e.g. G), then we need exactly two bits of
  information since two yes-no questions need to be answered: "Is it A or G?"
  (i.e. is it a purine?) and "Is it A or C?". (If the answers to both questions
  are "no", it must be a T.) Furthermore, if a position contains two bases (e.g.
  sometimes A and sometimes G), only one question suffices since a two out of
  four choice is equivalent to a one out of two choice. Therefore, only one bit
  is needed to describe a position in a binding site that contains only purines,
  but two bits are needed to describe a position that always contains adenine.


So yes, 2bits is the highest value you will see on the y-axis of a DNA sequence logo (proteins are a different story since there are more possibilities). In the one you show, the scale is from 0 to 0.2, but it's the same principle. 

However, you should think of logos as a useful visual aid, and not a way of getting rigorous mathematical information about a sequence. So don't put too much weight on the bit values. If you need that sort of data, calculate it from the alignment directly. 



[1] Schneider TD, Stephens RM., Sequence logos: a new way to display consensus sequences., Nucleic Acids Res. 1990 Oct 25;18(20):6097-100.