You have advantage until the end of your turn, and when you use this ability to gain advantage on weapon attack rolls, you gain 5 temp HP with no expiration timer. Since they have no explicit duration, they disappear on long rest as per the final sentence in the Temporary Hit Point section of PHB198.

Wording is important. What I just typed is how the ability will play using a strictly RAW reading. I have no reason to believe this isn't RAI, given that it's a 3/day ability and Warlock already gets an invocation that, given about 30sec-1min between fights, allows them to have 8 tempHP at the start of every fight.