If your goal really is "to be able to perform splits" (or to achieve maximal lower-body static-passive flexibility), and assuming that you already have the required range of motion in the hip joints to even do the splits (most people in reasonably good health without any hip problems do), you will need to be patient. Everyone is built differently and so the amount of time it will take to achieve splits will be different for different people (although SynerStretch suggests that it should take about two months of regular PNF stretching for most people to achieve their maximum split potential). The amount of time it takes will depend on your previous flexibility and body makeup. Anyone will see improvements in flexibility within weeks with consistent, frequent, and proper stretching. Trust your own body, take it gently, and stretch often. Try not to dwell on the splits, concentrate more on the stretch. Also, physiological differences in body mechanics may not allow you to be very flexible. If so, take that into consideration when working out.
First of all, there are two kinds of splits: front and side (the side split is often called a chinese split). In a Front split, you have one leg stretched out to the front and the other leg stretched out to the back. In a side split, both legs are stretched out to your side.
A common problem encountered during a side split is pain in the hip joints. Usually, the reason for this is that the split is being performed improperly (you may need to tilt your pelvis forward).
Another common problem encountered during splits (both front and side) is pain in the knees. This pain can often (but not always) be alleviated by performing a slightly different variation of the split.
For front splits, the front leg should be straight and its kneecap should be facing the ceiling, or sky. The front foot can be pointed or flexed (there will be a greater stretch in the front hamstring if the front foot is flexed). The kneecap of the back leg should either be facing the floor (which puts more of a stretch on the quadriceps and psoas muscles), or out to the side (which puts more of a stretch on the inner-thigh (groin) muscles). If it is facing the floor, then it will probably be pretty hard to flex the back foot, since its instep should be on the floor. If the back kneecap is facing the side, then your back foot should be stretched out (not flexed) with its toes pointed to reduce undue stress upon the knee. Even with the toes of the back foot pointed, you may still feel that there is to much stress on your back knee (in which case you should make it face the floor).
For side splits, you can either have both kneecaps (and insteps) facing the ceiling, which puts more of a stretch on the hamstrings, or you can have both kneecaps (and insteps) face the front, which puts more of a stretch on the inner-thigh (groin) muscle. The latter position puts more stress on the knee joints and may cause pain in the knees for some people. If you perform side splits with both kneecaps (and insteps) facing the front then you must be sure to tilt your pelvis forward (push your buttocks to the rear) or you may experience pain in your hip joints.