The journey across the Helcaraxë was difficult and perilous, but how long did it take to reach Beleriand?
Around twenty years.
In the earlier version of the Annals of Aman in History of Middle-Earth V, where it is noted that each Valian Year (VY) "is as ten years of the Sun now are", we have:
VY 2998-3000: Now Fingolfin [...] won their way at last with grievous losses and with minished might into the North of Middle-earth.
So it took two Valian years, or twenty years in the usual reckoning.