Literally every guys wonders how soon he should call the girl after he met her and go her phone number. This is an important question, and I believe that a different answer applies depending upon how you met the girl and a number of other factors. Below are some of the most common situations and corresponding suggestions of when you should call the girl if you met her in one of those situations:

1. You had a very short interaction with a woman. Perhaps you met at an elevator or in line at a coffee shop, exchanged a few words and a few jokes, and you got her number before both of you had to rush back to your own business. In this case, calling a girl 2-3 days later would be perfectly appropriate. Call her too soon and you run the risk of coming across as needy or desperate. Wait for 5 days or long, and she might forget who you were or might start resenting you for playing games, and make not mistake about it - most women have heard about the "rules" and will know if you "play" by them.  

2. You met a girl at a bar, lounge, or a restaurant, you had a great conversation, and it is obvious that you really like each other. In this case you should not wait or make her more than two days. If she liked you as much as you think she did, she will be anxious to hear from you, but if you wait too long, that positive feeling will transform into doubt or, again, resentment towards you for playing games.  

3. You met a girl online, had an interesting e-mail exchange, and you asked her for a phone number. In this case, you can simply tell her when about you will call her (over the weekend, or tomorrow evening, etc...) and stick to your word. It will make an early good impression if you show that you are responsible and keep your word.  

4. You met a girl at a bar or club, you had a great chemistry and connection and you spend many hours or even the whole night together. Whether or not you went all the way, you should call her the next day, or if you hung out till the morning, you should call her later that day as a matter of courtesy, if you are interested in seeing her again.  

5. You have been introduced to a girl by a mutual friend at a house party or in a similar setting. You talked for a while, enjoyed each other's company and you got her phomber at the end of the evening. Again, waiting for 2-3 days before you call her will be appropriate. 

As you noticed above, I do not really recommend waiting for longer than 2-3 days before you call a girl, as waiting longer is likely to make more harm than good to those initial stages of your communication with her. The only time when I suggest that you wait longer than three days is if it is obvious to you from your conversation with a woman that she had some bad recent experiences with guys who were very needy and desperate and who needed way too much of her  time and attention. In this case, that woman might actually enjoy having a little more space and a little less pressure from a guy who she just met.  

It's also important to note that  the more you liked each other during the initial interaction, and the better the connection you established back then, the less worried you should be about when to call her, because if she liked you, it doesn't really matter all that much when you call her, unless again  - you make her wait too long.