I run out of both at the same time. xD This is because I use the "2 in 1" kind of shampoo/conditioner. I'm not sure if a person's hair can truly respond to both things at the same time, but it's a convenient option for lazy dudes like me anyway. :P
Conditioner-I rarely use shampoo because most drugstore shampoos have very shitty ingredients that strip my curly hair and make it dry. Plus I have a lotttt of hair, so, I use a lot of conditioner. So, always conditioner.
I use a two in one. I run out of shampoo and conditioner at the same time.
I also use a three in one, so I run out of shampoo, conditioner and body wash at the same time.
When you use a three in one, you can squeeze a small amount into your hands and shampoo and condition your hair and then when your hands get really foamy, wash your body. It saves a lot of time, soap, water, and money. You're also saving a lot of room. Instead of having three or two bottles of different washes, you have one with all three mixed in one. Those also work great for a bubble bath if you love soaking in a nice, hot, deep bath.
Mr. Organised here - There is two of everything in my bathroom and when something runs out, I start on spare and buy a new spare next time I am in the shops which is every day so I am never 24 hours without two of everything.
Usually conditioner. My hair is very thick and a little curly/wavy so it tends to be dry. I condition the daylights out of it. Because I use multiple shampoos, I don't usually run out of those very quickly.
If you run out of shampoo because you use it more than conditioner I'm gonna pray for your hair x. x I don't think I've run out of my second bottle of shampoo this year lol and I get the little ones in the "ethnic section"