I went from 1.025 to about 1.008 in about 3.5 days and it didn't seem to bother the fish at all. I've read people have dropped it faster without any problems as well. From what I've read, increasing the salinity at a slower rate (ie. 7 days) is more important. Here's a link I found helpful:
http://atj.net.au/marineaquaria/hyposalinity.html
Best of luck!
