There's a lot of differing information from research. What you're doing now will make them longer, but if you stretch them for shorter periods each and every day, you'll make faster change. You may also want to try PNF contract relax, which is typically more effective.
Keep in mind that poor tibial nerve or common peroneal nerve mobility can also cause symptoms like tight hamstrings. If you have symptoms in your calf when you stretch, it's usually this. You can still gain range by stretching, but just don't push into any numbness, tingling, burning sensations.