Leafs will mostly always float, pellets will eventually sink.
5-14 days for the dryhop is typical, but not a rule. I personally like the 0.50 to 0.75 oz. ratio of dryhops per gallon of IPA or IIPA.
Some brewers even do a multi-stage dryhop, whereupon smaller additions are added and subsequently removed in 3 or 4 installments, in 3 day increments. If your processes are good, and you avoid oxygenation, this method will provide the most aromatic of IPAs. Though, as previously stated, amateur homebrewers shouldn't do this without some know-how since oxygenation is a major risk.