Welcome. I'm really into cut flowers and love lilacs. I have several references and only had time to take a quick peek at "Woody Cut Stems for Growers and Florists" by Lane Greer and John Dole. Under Comments, the first sentence states, "Dark-colored varieties will fade when forced indoors. If branches become wilted, they can be recut and placed in 110 degrees water." Also, if you have a water softener, your flowers will not last. I use a pitcher with a filter for all the water I use in cut flower arrangements. Always recut the stem on an angle so there is more cutting area to take up water. In the past, I have also cut vertical slits in the cut stem to help it get water. I cut mine and place into warm water and then set aside in a nice cool dark place (not the refrigerator) on my shaded porch before I arrange in a vase with flower food water. I hope this helps.
You may want to also check out resources for cut flower growers as they sometimes post tips on cutting. The book I listed also has additional information on cutting lilacs; however, it looks like 5 days is considered a good length of time. They also advise removing all leaves except those right below the bloom.
