If you take a perfectly health joint and do not move it for 8 weeks, the joint will sustain permanent damage.
Not being able to move your jaw is a medical emergency! If you are not able to open your jaw all the way, you need to seek treatment NOW. After 4 weeks of not opening your jaw, you have lost 50% of the height of the cartilage. This is still reversible after 4 weeks.
After 8 weeks, there is permeant damage to the cartilage. This will progressively get worse over the next 6 months, leading to a completely fibrosed joint. From the time your jaw becomes locked (stuck), you have 4-6 weeks to get it moving again if you want to maintain healthy cartilage. The first 4 weeks are critical. You need to be sure you are with a doctor who understands the urgency of the situation.