"We have not started planning," Simpson, 30, said during NBC's Christmas in Rockefeller Center live telecast Tuesday night in New York City. "I'm trying to get through all this promotion … Who knows what will happen?"
"I personally have no idea. I have not set a date and I can't even think about putting together like table settings and all that," said Simpson, who attended the annual tree lighting with Johnson, mother Tina Simpson and friends. "We're just going to enjoy the engagement and we'll get married the moment we feel like it."
While she says she isn't sure when she and Johnson will become Mr. and Mrs., she told reporters that the big day would most likely take place within the next year. "By next Christmas, hopefully … we don't have any plans but I'm pretty sure by next Christmas we'll be married."
Whenever the nuptials takes place, Simpson tells PEOPLE that she wants the wedding to be "simple."
Johnson, 31, proposed on Nov. 11 while she was watching Parenthood at home in Los Angeles, surprising her with a five-carat ruby and diamond sparkler from Neil Lane – which she called incredibly romantic and brought her to tears.
And that was only the beginning: "I honestly feel every day is a romantic day with him. He even said [earlier] to my mom and my friend, 'I can't even believe that's my fiancée.' My mom's heart was melting. It's so romantic to be here and celebrate with him."
As for holiday plans, she said during the telecast, "That's up in the air … We spent Thanksgiving here [in New York], so … I think we'll split it up. I think we'll do the Boston thing, because he's from Boston." Texas may also figure into the travel plans, she said.
Simpson also told PEOPLE that Johnson gave her a "thumbs up" for the vegan Thanksgiving she prepared him. "I attempted," she said with a chuckle.
She added, "There's lots to be thankful for this year and a lot to celebrate in the holiday season."