How much to put in a 529 plan.

With the next morning being the start of a new year I will be putting some money aside for my 2 kids (5yrs and 3 yrs). I have opted for the Utah 529 plan wanted to know if this is a good choice and how much money should I put aside each month.
I tried to get this answered from the internet but got even more confused.

Advise and response appreciated.