That's going to depend on the base malt you are using; you want to ensure that you can convert the maize as well as the malt. Many cream ales (including my own) are made with 6-row malt precisely for this reason - your typical American 6-row malt has a diastatic power of up to 160° Lintner, whereas American 2-row tops out at 140°, which is still quite a bit higher than most other base malts. This article gives a general range for the diastatic power of several malts, and how to calculate the conversion capability of a given grain bill.
That having been said, I doubt you'd want to use more than 50% at most, even if the base malt can convert a greater percentage. Otherwise, the body would suffer too much even for a lighter style such as this. My own cream ale has both maize and rice, as well as some carapils, but the most of any one adjunct is a little less than 23%.
I done a cream ale a while back using 25% maize and 25% flaked rice, 45%Maris Otter and 5% crystal 75 and it came out superb.
At 5.4% its a wolf in sheeps clothing too, very light and easy drinking but a fair whop for a "lighter" style beer