Q: what is the maximum length of an hdmi extension cord I can use with the Belkin Mini DisplayPort to HDMI adapter

That depends on the devices you are using and the compliance testing results of the particular HDMI cable.

There are no published maximum length specifications for HDMI cables. Signal quality is not an issue; it will either work or it won't.

In broad terms 25 feet or less is not a concern, more than 50 feet may be a challenge.

Don't spend a lot of money on cables. Purchase them from Monoprice. If it doesn't work they'll refund your money.