I've done salt baths on my golds ... not rainbows ... so read the following info with a grain of salt!
3% is the right amount for golds. How long depends on the fish. At that salt level, you need to closely watch the fish. When they first go in, they will dive to the bottom. That is normal, but if a fish appears passed out, remove to fresh water immediately. Weak fish might stand the salt bath for 30 sec or less, but the treatment will need as least a few minutes to be effective. For golds, you are supposed to wait till the fish poops (high salt will purge the fish) so you get rid of any parasites in the gut. I've gone as long as 20 minutes with my golds, although 10-15 minutes should be plenty long for a salt bath.
BTW it may be prudent to start with lower salt levels for the first bath to see how the fish reacts. At lower salt level, you need to leave the fish in longer to treat, but I think it is gentler for the fish. I started at 1% the first day for 45 minutes or so, then increase to 2% next day & finally 3% the 3rd (shorter time with higher concentrations). I noticed a major improvment even after the first bath of 1%, and the ich spots were all gone after the 3rd day. I am not sure if increasing past 1% is actually needed, but 3% was what is recommended so i went up to that level just in case there is any resistant bugs left.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.