^^Read trip reports for the personal experience, for your first time you should start out doing small bumps until you gauge how much you need of the stuff you have to feel it, and it lasts about 20-35 minutes so plan on doing a line at least a couple times an hour, and that's if you have good shit.
Most cocaine is hardly pure and wont feel like it's supposed to. If you feel really up, stimulated, talkative, or anti social, and especially if the high last more than 30ish minutes, it's probably cut with amphetamine or has a large % of impurities from the production process.
If it feels nicely euphoric, chilled out, and happy/clean, it's probably pretty good shit. You won't feel like running a marathon or be bouncing off the walls. You will be very happilly high, you won't feel like shit, and you would be happy doing whatever whether it's dancing or watching a movie. you won't feel the need to get up and go. If your entire face feels numb it's cut with novacaine or another similar topical anesthetic. Cocaine should only make your nose feel numb, not a large part of your face.
If based on this you suspect your coke is impure, there is a cocaine purification thread here that you can search for.
/thread.