Depending on how I do my wng's, they last 1-2 days. If I'm being lazy, I cowash my hair in the shower, leaving it in to detangle. I rinse, add more and mix with PC, detangle one more time (both wide tooth combs), and rinse. I add KBB Hair Milk and seal with oil, if I feel like adding gel--depends on laziness factor--I do that by section. Normally I'll do that every day, 2 if it's a weekend.
If I'm being careful, I'll wash and DC. I'll use Teri's method of loading conditioner (literally googobs) and detangle with my Denman. I'll then section my hair and add gel to each section and do the smoothing method. I'll hop under the dryer for a little while, and then I'll pseudo-pineapple my hair before going to sleep. I can get at least 2, sometimes 3, days out of that before washing, and without a major detangling session.