Haven returns to Syfy with its second season premiere tonight at 10pm. Last season, Haven hit its highs in its series premiere on July 9 (also Friday at 10pm) with a 0.7 adults 18-49 rating and 2.34 million viewers. It was in a range of a 0.4-0.6 adults 18-49 rating the rest of the season, scoring a 0.5 adults 18-49 rating in its season finale on October 8.
Perhaps worth noting for prognosticating purposes: when it premiered last summer, it had the summer premiere of Eureka as its lead-in, in its season finale in October, it had Friday Night Smackdown as its lead-in.
My guess is a 0.5 adults 18-49 rating.