If you allow the knees to bend, the hip flexors work less and the quadriceps muscles work more. Try kicking both ways and you will notice the difference. Over a period of several weeks, work up to eight to 10 periods of 30-second versicle kicking. Do this three times per week. This often depends on the range of motion of your foot when you begin.
Many swimmers notice some benefits within one to two weeks.
If you stretch aggressively and regularly, you will improve for many weeks. The versicle kicking exercises will start to pay off in two to three weeks.