How long is chamois cream meant to be effective?
It massively depends on the brand/consistency of the cream, how much you use, and the weather conditions.

For me, a small amount of Assos cream will last 4-6hrs in normal conditions, but if its particularly hot (and hence sweating a lot) or if its rainy it lasts much less.

There are some products that are much more durable (morgans blue solid for example), but are also not easy to wash out of shorts again afterwards.

I never carry any with me on a normal ride, but it is always in my day bag if i'm doing a supported tour.  If I was doing something properly long like PBP or LEL i'd definitely take some.