I would argue (based on Karen Adolph's data) that, if possible, more often would be ideal. When children are learning to walk they cover 29 football fields a day- it takes epochs of practice to develop a skill.
Depending upon patient diagnosis and patient tolerance.
Speed: to stimulate CPG, must be faster.
Frequency: Research shows improvement with as little as one treatment session per week. 3x/wk is average.
Ideally would like to walk patient at speed slightly faster than what they are able, increasing amount of time and speed and decreasing amount of weight supported as therapy progresses.