Women keep their phones on them all the time. If they are interested in a dude or interested in having a conversation with a dude they will text back ASAP. Doesn't matter the situation. If they don't care whether they have a conversation or not with you, they will text back when they feel like it because you're shyt to them.
If you are attractive to them they will text back immediately. If you're not good looking then you aren't a priority and therefore she will text back when she wants. Most likely after she already texted the attractive dude and/or person(s) she's interested in having conversations with.
I suppose you can't blame women for that mindset, but the smart thing to do is delete the number immediately and move on to the next person.