My family and I left out of LGA for a 6:30am flight on 05/25, the Saturday of the Memorial Day weekend. I allotted about a half an hour to get there (a normal drive at 4am-ish should olny take about 15-20 minutes) and gave us about 90 - 120 minutes for security/check-in. Even though I had stayed up all night washing clothes and packing, I still managed to run about fifteen minutes behind schedule.
I did however peek on the internet before we left and saw that I could do my airline (Delta) check-in over the internet! If it is available to you, definately take advantage of it. The taxi arrived on time but I was still scrambling to make sure I had everything. About five minutes away from the airport we were stuck in, fortunately minor traffic. Once we pulled up to the the departing drop off area, I was glad to have already check-in. All the lines were sooo looong; even though we only had to wait on the security line, we were still concerned about not having enough time before boarding! As the security lines grew longer, airport personnel did there best to control frustrations by allowing those who were departing soon to move to a newly formed lined, which, in turn, made the other lines move a little faster.
So if you are traveling around a holiday, I would, regrettably, allow about 2 hours for check-in and security. HTH
P.S. If you are returning to LGA and it is after 10pm, do not have your car service pick you up at the departing drop off area, in an attempt to avoid the baggage claim area crowds. They close the departing level to traffic after 10pm (at least when we were there they did) as a security measure, since there are no more departing flights at that hour.