Short answer: The next day.
Long answer: A good rule of thumb is to followup with a girl the next day from when you got her phone number. So if you got a girl’s number on Wednesday, followup on Thursday.
The whole “wait a couple days” is BS. Apply the old school sales idea here; close it when it’s hot. Keep the emotional momentum going while you can. That’s why you want to follow up as soon as possible. She might not feel the same about you a couple days later from when you guys met.
I like to text girls as soon I get their number, especially when I meet them at night. So if I get a girl’s number Saturday night, I’ll text her the same night. I’m texting while I have the momentum and she will get used to texting me. Then the next day I’ll followup.
If I get a girl’s number during day game, I’ll followup the next day with my first text message or call.
I like texting first before calling, because it is so low-compliant. It doesn’t ask much for her in return and I can calibrate how “into me” she is. If she is really into me, I know I can call her and she will pick up. If she is not that attracted, I can gradually build it up over text messages and then call her later. There are exceptions of course, but generally I like texting first.
Another tip: Most girls don’t do much on Sunday, so that’s the perfect day to call all your girls. For me, Sunday is like maintenance day. I’ll call all the girls I haven’t seen in a while and with whom I want to stay in touch with.