For awhile I slowly improved my flexibility when I joined team, but I seem to have hit a point where I won't stretch any farther!
You have hit a plateau in the protocol that your stretching is being programmed. The only way it's going to progress is by moving beyond it or changing it up. It may also help to work a few stretches that stretch any muscles that are really tight and limiting your flexibility. Try half splits in front or side, pidgeon, butterfly, or these:
Glute Stretches . Check internal/external rotation of the hip.
Some research has shown that it is necessary to stretch up to 30 minutes in a position to faciliate muscle elongation. This is done in PT studies. I know that in ballet they frequently hold stretches for 15-30 minutes whereas most gyms rarely go over 3 minutes if 5-7 in the off-season.
That being said, I think it's something that has to be worked up to. It's gonna suck.
You know the whole diatribe, " Relax into the stretch, breathe. "
Build up slowly. Don't bother to stretch the next day as you will probably create enough tissue damage in one session that it may take a few days or a week to fully recover, especially if you get into the 7-15 minutes duration.
I do something like:
30 second hold (basically warmup) 1 minute, 1 minute 30 seconds to 2 minutes, 3 minutes for the first session; perhaps even omitting the last 3 minute hold. Don't stretch again for similar duration until your legs feel fresh. Obviously, it's best if the tissue is warm before stretching which is generally the case at the end of practice or after a warmup of some kind.
Next session add the 3 minute hold, and then possibly a 5 minute hold. At the session after that, you might repeat the protocol for another couple of sessions unless you feel ready to move on. If so, add a minute at a time, or 30 seconds at least.
I would not do more than one session a day because more than likely the tissue will not have recovered within time in between ( from 4-12 hours ) to be ready to then stretch again for such a duration. It probably is alright to stretch for up to a minute or 90 seconds after one of these session but it will depend on how much tissue damage was done.
Again, it's probably going to be uncomfortable as all get out. I wouldn't be surprised if a gymnast cried or tried to tear my hair out or fight against while doing so.