The Twilight heartthrob was devastated when co-star K-Stew confessed to cheating with Snow White And The Huntsman director Rupert Sanders earlier this year.
But despite the affair, Rob gave Kristen another chance and the pair hit the red carpet together for the premiere of the Twilight finale Breaking Dawn - Part 2.
Apparently though, the rekindled relationship was only on a "trial basis", and R-Pattz has yet to decide whether it'll be a long-term thing.
According to Now magazine, Rob has given himself until Valentine's Day to make his mind up.
A source told the mag: "Rob's planning to skip most of the upcoming awards parties and spend January focusing on his relationship.
"He's set a Valentine's Day deadline to decide whether he's in it for the long run. So there could be a proposal on the cards - or a decision to end it."
One person who reportedly won't be happy if R-Pattz pops the question is his sister Lizzy.
A gossip told The Sun newspaper that the siblings have been rowing over his decision to take K-Stew back, saying: "They were out drinking in Barnes, where Rob grew up.
"Lizzy was giving him grief about taking Kristen back and said she's always hated the way he's been treated by her.
"He and Kristen spent the festive season apart partly because she's terrified of facing Rob's family."
Stewart recently confirmed that she had signed up for the SWATH sequel but it's thought there will be a new director at the helm.