When should i follow up with interview?

If they promised that phone call by end of the day it is a shame that they didn't fulfil the promise . Even if it was to tell you they had selected someone else or there was a delay . I suspect they have offered to someone else and are waiting on them to confirm acceptance before they tell the other candidates . Give them till tomorrow (or Monday ) and if you have not heard anything give them a call . If they have offered to someone else you can ask what made the difference so you can ensure with next job interview you can address any issues . Even if you haven't got this job , which will obviously be disappointing , the fact you got selected out of 300 for interview and had a long interview should tell you that an offer of employment should happen soon Good Luck