It's pretty much minecraft magma, it will keep flowing until you cut off the source block, but when I think about it maybe I will have it cut off a branch if you dig a hole in a horizontal branch higher up the chain. Could come up with interesting scenarios for how people deal with the magma.
**EDIT
Took into account puggsoy's suggestion and tweaked the magma flow
**EDIT
Took into account puggsoy's suggestion and tweaked the magma flow