In the Perils of the Warp table under the point Rending the Veil it says:
The air vibrates with images of cackling Daemons and the kaleidoscopic fabric of the Warp is rendered visible to mortal eyes. All sentient creatures within 1d100 metres must test against Fear (2). The psyker must test against Fear (4) instead. This effect lasts for 1d5 rounds.
Does this mean you have to roll against Fear every turn for 1d5 turns?