How Long Does an Oxbow lake formed?

It's formed when a river changes its course, leaving a corner isolated from the main flow. When a large river flows round a curve, it scours out a deep hole. When that hole continues to hold water, that stranded part of the river is called an an oxbow lake.

i think it is when you have a new opening to and already flowwing valley- sort of like a new rout for the water to flow but the new rout eventually joins into the original flow if you get what i am trying to say.

that was my non-scientific explanation here is a more sencible answer:

An oxbow lake is formed when a river creates a meander due to the river eroding the banks through hydraulic action and abrasion/corrasion. After a long period of time this meander becomes very curved, and eventually the neck of the meander will touch the opposite side and the river will cut through the neck, cutting off the meander to form the oxbow lake.