I'd probably put at least the maximum state-tax-advantaged amount into the 529 each year, because it is a relatively small amount. Up to you to understand whether the deduction or credit is better for you.
After that, it depends which rule of thumb you want to follow:
a) the stock market, on average, goes up more than money markets so invest the 529 amount in some stock fund
b) the stock market sometimes goes down, so put money you need "soon" into safer investments (e.g., money market fund in the 529).
The definition of "soon" is best judged in hindsight. Some will say "3 years", others "5 years", etc.