Halving cassoulet recipe cooking time
As several comments have said already: halving a recipe generally doesn't affect the cooking time very much. Depending on the recipe you may have to keep an eye to ensure that the increased surface area (relative to volume) doesn't cause it to evaporate too quickly or develop too much crust, but for the most part the time it takes to make something hot and allow chemical reactions to occur is about the same regardless of volume.

You're probably thinking of larger things like roasts and whole birds, where there is a minutes-per-pound estimate in cooking. That's a matter of radius, where you're just trying to get the thing to heat all the way through. Even there, minutes-per-pound is not a very good estimate. 

For a casserole, braise, etc., it will reach temperature fairly quickly, and you're waiting for chemical reactions to occur (collagen gelatinizing, starches breaking down). That's the category your cassoulet fits in.