i dont think it matters how soon you open. Timing is important in the sense that you probably dont want to hit them up in the early morning or whatever. try to catch them when they're not busy (sunday afternoon)
as for openers...thats a whole nother animal. Everyone has different opinions and theres many approaches...basically you can go with the safe, friendly approach, or you can get sexual very quickly (high risk, high reward)
My fallbacks when i can think of anything better are:
1) "You're cute." - she matched you, so you already know you find each other attractive. this is also a good alternative to the creepy "you're sexy" or "you're really hot" that most guys send. This one might be better used by guys who are very good looking as it is direct (not saying you're not), but try it and see what works. Usually they reply with "thanks, so are you" or just "thanks"
2) "name!" Just put an exclamation point after their name. for example "Carly!". This is good because it's simple, doesnt show that you sat there for 20 minutes constructing this elaborate, pathetic monologue. Its more personal and less boring than a "what's up?" or "hey"