Lindsay Lohan is reportedly set to extend her stay in rehab.The troubled actress - currently in residence at Malibu's Promises rehab center - has decided to stay until she has fully recovered. A source told the New York Post newspaper: "She is staying in Promises for longer than a month. We don't know how much longer because of her work schedule, but she is taking rehab very seriously this time and not messing around."The 'Mean Girls' actress was initially set to leave Promises - which normally lets clients stay for a month - at the end of this week.On Monday Lindsay cancelled her 21st birthday party at Las Vegas' PURE nightclub.PURE representative Kate Turner said: "We support Lindsay and wish her the best as she is taking care of personal matters at this time."Due to the unique circumstances Lindsay's birthday party at PURE Nightclub has been cancelled. We think the world of her and look forward to working with her in the future."Lindsay, 20, entered rehab for the second time this year following her arrest for DUI (driving under the influence) last month.Police found a "usable amount" of cocaine in the vehicle Lindsay has been driving.