Honestly, I think that this really, really depends on your LO's personality. At 13 weeks I think that some babies are able to last longer on their own and some need lots more attention. I would follow her cues and if she's content fine and if she seems to need you that's ok too. I've also noticed that my LO goes through phases -- earlier on she was very happy to sit on her play mat by herself and be happy -- as she got older, she needed more interaction, then less, then more....at the present time, she's strolling about the upstairs of our house checking in periodically, but mostly playing by herself. I would just follow her cues - she'll let you know if she needs you, and as long as she doesn't seem to need you every single minute of every single day, I wouldn't worry too much.