If I'm sustaining a single psychic power do I need to take a test every turn or is it only if I'm doing 2 or more, the rulebook's kind of contradictory on the subject?

no matter how many powers you sustain you never make checks every round just to activate them, but if you sustain more than one all sustained powers become weaker (your psy rateing goes down in regardes to sustained powers) and it increases the dangers of perils of the warp when you use other powers whail sustaining them.

Librarian's right. You only make the check when you first cast the power. But each power you sustain decreases your effective Psy Rating, up to the point where you can't sustain anymore.

Though, it's a bit ambiguous as to whether the effective psy rating on a power you were already sustaining decreases when you start to sustain a new power, or whether only the new power has a decreased psy rating.

Tengentally related: Nothing says or even suggests you can't sustained powers used at the Push power level. My melee-psycher spent half the campaign with an indefinitely-sustained pushed warptime up, only ever recasting it after having to sleep.

I dunno if "reduces the effective Psy Rating for each power by the number of powers being sustained" is that vague. For EACH power by the number seems, to be, clear. But hey, not everybody is me, and thank the gods for that!

That said, reducing the Psy rating of previously cast powers seems more sensible to me, since otherwise there's no incentive to not be sustaining as many powers as you can, cast in whatever order their rating is most important. By decreasing all powers, you both give the dilemma of either prioritizing having one potent effect or many weaker ones, and discourage an excessive stacking of effects.

As for the topic of sustaining indefinitely, yeah, you can do that. That said, even free actions require you to be able to spend actions, and characters who are even temporarily stunned lose that ability. Against combat psykers, a power maul set to stun is often more deadly than one set to kill. Also known as "learn to hate Shocking".

And yes, you can push and start detracting from that. Downside of course being that you're pushing, so there's some risks involved. Of course, you could do it in other ways, like Boon of Tzeentch, which is brutally good for stuff like that. It, however, also discourages doing it too much. Since even with stuff like Protean Form and Telekinetic Shield up to make sure you don't get little or no damage, that stun test might eventually do you in regardless, or risk losing all those powers and having to go again.

There is, of course, the less mechanical downside of sustaining some powers. While stuff like telekinetic shield, mind link and preternatural awareness probably have few or no obvious effects on the user, stuff like Abhorrent Ward, Host of Fiends and, yes, Warptime are far more obvious.

A guy with Warptime up, especially at higher levels, is moving at superspeed to the rest of the world. Zooming around like that could, especially for anybody who's even been victim to a combat psyker, be pretty clear signs that they either plan to initiate or walk around expecting combat. Not exactly great for those fragile allegiance negotiations.

Then there's the purely mundane downside. On Warptime, the world moves in slow motion. Every conversation is drawn out, tedious and incredibly slow. Every word you say is blurted out at superhuman speed, and people are unlikely to understand what you're saying unless you take great care to draw out every sound and syllable. Every task your friends undertake is ten times slower, and you have to stand their waiting as they wade through the swamp of time to get it done. Hell, even just walking with them would get unbearable, since your casual walk is their all out sprint.

If I had a player who constantly sustained warptime, I'd point this out to him, and expect him to roleplay it appropriately.