I tried to setup 2 client and wanted to see how long they can keep active with the video call

The 2 clients are Linux UBUNTU 12.xxx + ICA 13.1; Lync server/client 2010

I connect to 2 different desktops from the 2 clients, video call one between each other

It kept alive for about 2-3 hours, and then the video call is terminated by itself with message e.g. "the user has no response"...... the desktop connections are fine

I tried at least 3 times and it is always the same result

Any idea if this is one of the lync limitation e.g. video call can only stay alive for 2-3 consecutive hours?

There is no hard set limit.  The video call could potentially go on indefinitely assuming no outside (ex: network or client issues) interference.