My general rule of thumb is 9am-9pm. There are exceptions, however. My mom, who I call sometimes when I get home from job #2, anywhere from 9:30-10pm. But she's a night owl like me, so I know unless something is really wrong, like she's sick, she's up. And in the am, I will sometimes call her starting around 8:30 - again, I know her scheudle, and while she may not be 100% awake, she's up and willling to talk. and if not, she has no problem telling me so!
I ahve other friends who are early risers, and will sometimes call me early. Sometimes I'm up, but not in the mood to talk, so I let it go. And call back later.
But for people I don't know well, or don't know their "habits" 9 to 9 is what I try and follow.