from what I remember learning, one should not daven for a person who is not sick. I personaly think that when davening for a name recived by text, a week is good. If the condition changes for the worse, the person who started the text will do it again.
Text messages asking people to daven for Yosef ben Rochel (as a joke) should be stopped. Davening is serious thing. Making people daven for no reason IS ASSUR.