With BC, OW now out i feel psychic powers on Rogue Trader are bit outdated.

I found it hard to decide whenever or not sustaining particular power is free, half or full action?

how have u delt with this? or is there allready answer for this guestion? then please help fellow gm

It's apparently free, since you can sustain up to 3 powers, and you don't have that many half actions

EDIT: Can someone tell me how long one can sustain a power?

As there are no rules, and Astropaths are meant to be top-tier Psykers, I've never required a test. Some powers say they only work while the Psyker is concentrating (e.g. Mind Probe), so if they go out of rang ethen the power stops working. Also, Astropaths can theoretically sustain up to their (Psy Rating - 1) in powers. The important thing to note is if your Astropath is sustaining powers and he for whatever reasons suffers Psychic Phenomena, say from being exposed to some unknown Xenos drug, then they take a cumulative +10 for any power they are sustaining at that time.