I have my ipad2 and my iPhone on all the time. I frequently have text conversations with my daughter throughout the day.
My iPhone usually receives a text, a second or two after the iPad. But sometimes it can be up to 4 hours before a reply comes through.
I have just received a text a few seconds ago (8.38am) sent by my daughter at about 11.15pm last night. And three times during last night, the phone woke me up with random text replies from her, sent hours ago.