Go for it whenever you like. The salt bath is great for helping to heal the stitches. I'm not sure about roids - I thought I got them after having P but turns out it was a fissure (which keeps recurring, the bastard), and salt baths were fabulous for that too. I can't think how it would be a bad thing for you.
I was never advised that there's too long to spend in a salt bath, but I guess be guided by what's comfortable for you.
GL, I hope it all heals up quickly for you and congratulations on your LO