I you use a LM317T configured for constant current, and the current is low enough, it will not turn off but it will never overcharge the batteries. The charging current depends upon the AH capacity of the cells.
You say it is 6V....does that mean there are 4 cells or 5 cells? The reason I ask is that Ni-Cad cells are rated at 1.2V, but during charge they approach 1.5V per cell so 4 cells would be 6.0V during charge or 4.8V when not charging. 5 cells would be 7.5V during charge or 6.0V when not charging.