What is the maximum income amount when the property tax credit no longer becomes available? I have never heard of this before.

Per the instructions for Form CT-1040 the maximum credit is $300.  It is figured on Schedule 3 of your Form Ct-1040.  There is a reduction in the credit, according to the Connecticut State Department of Revenue website, it states as follows: 

"Depending on the amount of property taxes paid by you to a Connecticut municipality and your Connecticut adjusted gross income, the amount of your property tax credit may be reduced or you may not be entitled to a credit".

I have provided a link to the instructions below.

This table is in the instructions for Schedule 3, which is the Schedule that determines your property tax credit:

Click here for Instructions to CT-1040; See page 14 of this instruction under Schedule 3.