For some people it does take much longer. You just have to persevere! Be sure to warm up REALLY well, stretch little and often, and vary the stretches you do... and don't give up! :)
Congratulations! :)
You can also stretch box splits by sitting in a straddle, and doing side/ forward stretches, while moving your legs further apart. You can also do this against the wall for an extra push...
If you do it 'sliding down', once you get past a certain point you should be able to...
For both front splits and back splits, it can help to do other stretches as well as the actual splits. So for front splits, if you try lunges with a straight leg behind (don't let your knee go past your foot), and hamstring stretches (toe touches with flexed/ pointed feet). Box splits, you need...