I second Bamenwubu’s suggestion that you check with your teacher to make sure you have the proper alignment in the movement (back foot open 45 degrees to the back corner for the bulk of the movement, right knee in line with the right toe and not falling inward, left foot straight ahead and so forth).
But that said, it’s fine to maintain a high stance until your knee heals up—whether that’s next month, or ten years from now. The important thing is to not make the injury worse by over straining it. Build your knee strength (the tendons and muscles surrounding the knee) gradually. The more relaxed you are in your stance, the more qi will flow through the area, helping to heal the knee. But if you force it, it will be tense and the qi will stagnate.
Yang style requires that you keep to the ten essentials and conventions like toe/knee direction in the stances, shoulder-width bow stances, and empty stances with footwork on either side of a center line—but aside from those, there is a lot of room for personal variation depending on leg strength and general health. For example, a beginning student who is a senior citizen with a very high stance who can only do that movement by bowing forward slightly can be said to be doing Snake Creeps Down correctly, so long as they are adhering to the ten essentials, the correct intent of the movement, and have their footwork at shoulder width (no matter the length or depth of the stance).
On the other hand, a young student who is flexible enough to get into the lowest possible variation of Snake Creeps Down will be doing it incorrectly if they just bounce downward quickly (insufficient leg strength to go slowly and evenly), or if their right knee collapses inward, of if they have to stick their rear way out in order to get that low (tailbone not tucked under enough).
My advice: stick to the ten essentials and the footwork requirements but don’t worry about the depth of your stance—that will come in time. Go as low as you can without pain or straining your knee. You will improve more quickly for making haste slowly.
Sadly, I’m talking from experience here—I injured my right knee in January attempting to fa with incorrect footwork. I’m slightly pigeon-toed, which I’ve corrected in my form, but in the heat of push hands, I didn’t notice my toes were pointing in when I pushed my opponent. I ended up twisting my tibia and straining some of the ligaments. So mostly, I’m sticking to a 45 degree bend in the right knee (that’s what feels comfortable) for Snake Creeps Down until it gets stronger. I’ve also had to modify Turn Around and Left Heel Kick because the knee is still too unstable to do that spin. It’s a bit frustrating, but better than re-injuring it, which I’ve done a few times already on account of being impatient.
The upside of the knee injury is that I’ve been able to correct a lot of structural details in my form that I was unaware of when my knee was strong enough to support me stepping incorrectly. Because of the general swelling and pain, I had to develop the ability to listen to my knee and what felt right. I’ve learned that the knee can actually bend more deeply and support my weight with less pain if I tuck my tailbone under, and maintain a strict vertical alignment (central equilibrium and head top suspended). I thought I was doing those things—but the knee pain has forced me to see where I’m not. It’s made my transfer footwork much better (brush knee and push sequence, step up to grasp the bird’s tail, parting the wild horse’s mane) and also improved my stability when kicking. I’ve gotten a lot more from working with my knee and listening to what it can and cannot do than when I had a more adversarial relationship with it.
Anyway, good luck with it and best wishes for a speedy recovery (even if it is an old injury),