Usually you need 3 days of un changed gravity readings. That's a sign that fermentation is done and you can proceed to bottling.
And yes, you could experiment with dry hopping. The nice thing about hops is they are anti bacterial so no need to Sanitize, just throw them in. But not for too long otherwise your beer will have a grassy flavor. I would say a max of 2 weeks dry hopping. Try and find a nice floral hop like cascade for dry hopping.
Anything else you add needs to be Sanitized, don't forget that.