My rule is "Never send more than 3 text messages without receiving a reply." If they aren't responding, there is usually a reason. My husband is a chronic abuser of this rule. Whenever we get in a fight, he goes crazy and blows up my phone with constant texts and voicemails (to the point that I have to block him and change my phone #). He has it in his head that "the more text messages I send her, the more she will realize how much I love her," and that is definitely NOT how I feel! Sometimes, less is more. Give someone a chance to miss you.