Just until you can't see any of the salt grain any more. If you stir the water with them still in it you might stress them, anyways.
Don't worry too much.. I've seen a few other people on here add salt to their tanks before they knew they were supposed to dissolve it first, and others advised them to cup them and then dissolve the salt. I know it might be slightly difficult with two fish in the tank.
You also might possibly be able to scoop up the salt with a net or something and swish it around in a separate cup if that's easier? I've never done it, but it sounds doable.
Last edited by AyalaCookiejar; 12-02-2012 at 02:12 AM.