Game Shape is updated each Friday as part of the training update.
Game Shape is reset to Respectable at the start of the season. After about 3 weeks, players who play a moderate amount (50 to 80 minutes per week) will have either Strong or Prominent game shape, and as long as they continue to play in those ranges will stay at those top two levels, though they seem to change between the two more or less at random.
If a player plays more than that, there is increased chance of a drop. At 90 minutes, it might not drop. At 140 minutes, it almost certainly will drop. Similarly, a player who plays less than 50 minutes will tend to drop in game shape.