Ahh my expertise

The whole 30 min thing is normal, "girls that you want" woulnt give out their number online in 5-10 minutes when they dont know you, and girls you dont want might lol.
If you want to close it quickly find some girls with self esteem issues or something like that, if you want a quality girl put in the time to play the "game".
Simplest #close I use on pretty much all the girls online (and its worked 9/10 times for me) is-
(after the chat gets boring, and your running out of DHV's or things to say)
me:uh oh I gotta go to work soon (some time limitation thing, tailor to your needs)
her: ok then cya?
me: oh one last thing before i go...
her: whats that?
me: you txt like all the kewl kids do right?
her: haha yes
me: and unlimited, thats a must huh?
her: duh everyone has that
me: why dont you give me your number - maybe ill text you sometime
her: (she might give it to you here, but just incase) hmm. idk
me: Yea I guess your right, I do have to many people texting me already
her: oh really?
me: yea It's a exclusive thing, you have to be really awesome to txt me
her: haha suuure
me: try it, i g2g now so last chance
her: ###-###-####
me: alright i'll text you sooner or later
her: why not txt me now so ill have your number?
me: That's a secret

ttyl
The pro's of this are if she REALLY doesn't like you she'll eaither leave when u said "i g2g soon" or tell you no during the "you have txting right?". Theres no way she'll go "fk off you creep" and thats especially good on social sites where people can see your posts.
Obviously not every chat will be like this but it's really something to base it off of, theres a million other ways but this usually works for me