Guidelines for isometric stretching
For increasing flexibility, isometric stretching should be done at least twice a week, at a maximum of about four times a week. For maintenance, it should only take one good application per week. The isometric method is best for those stretches that are stuck at a plateau, so I would only recommend using isometrics for those stretches that you really need
greater mobility in. The splits are a good example of stretch that can easily plateau, isometrics work wonders for the splits.
Isometric stretching is not for everyone, to see if you are ready try it out! If you are consistently sore after every isometric application, if your flexibility is getting worse or not improving, or if you actually hurt something in the process of using it, you're not ready for isometric stretching! You are a wimp! Nah, but because isometric stretching involves the utilization of tension in extended positions, some people might not be prepared to employ them in their training; Moreover, it is usually not recommended for younger teens and children for this same reason. Depending on the strength of the muscle and the amount of stress the stretch causes, the damage from poor preparation can announce itself as muscle soreness or complete muscle tear! Finally, if nothing else, poor preparation will likely hinder results, meaning you could be wasting your time.
Gawd! Something is wrong, cause I tried it and I've been sore for days! It's painful; But I want the splits, is it safe to continue?
It's likely the lack of strength. You can try reapplying it and seeing if your body adapts overtime. If an injury has announced itself you're done for!