Credit: Kevin Mazur/VF11/

Don't count on a long engagement for Justin Timberlake and Jessica Biel.

After nearly five on-and-off (and sometimes rocky!) years together, the recently engaged twosome plan to tie the knot later this year, an insider tells Us Weekly.

"The big day's this summer at a private estate," an insider says of Timberlake, 31, who proposed to the actress, 31, in late December.

What will the stunning bride wear to become Mrs. Timberlake? "Monique Lhuillier is her No. 1 choice for the dress," the source reveals of the beloved designer, whose dresses have been worn by such A-list brides as Reese Witherspoon, Jessica Alba, Vanessa Minnillo and Shannen Doherty.

In keeping with the couple's uber-private M.O., it took Biel about six weeks to debut her engagement ring; she finally sported the rock (showing it off to Amy Poehler and Kristen Wiig!) at a Saturday Night Live afterparty at NYC's Ristorante Asellina Feb. 18. "It wasn't huge," a witness says. "It looked vintage and might have been custom made."