Guys, how long do you take to reply to a girl (if you have a life)?

I casually asked this guy out over fb but I haven't heard back from him yet - it's been 2 days. Not that i'm watching the screen 24.7 but my question is - how long should you give a guy before assuming he's completely disinterested? And guys, what are the reasons you usually don't reply to a pretty attractive (not being arrogant, i've been told) girl's msg to you? Is it always just he's not that interested? Thanks