Regretfully, you can't tell when the stoma will be at it's most active as we are all different.
For me, transition from eating to output is around three hours - for my partner it is around a day and a half.
Rarely will you "feel" the bag filling up - the stoma itself has no nerves. If you could, you would be awake all night, every night.
What type of stoma do you have - Ileostomy or colostomy?Andy