from what i've seen using the avatar. after about 4 pieces of food digestion takes about 30 seconds. nothing really speeds it up. but eating more after the initial four will reset the timer. belly rubbing just helps prevent indigestion if you've eaten past a certain amount
The timing depends on the size of the internals, but broadly speaking, you'll accumulate weight constantly while someone is inside. The weight gain speed is about equivalent to a round of food-digestion (4 pieces of food, or 2 slices of garlic bread) every 145-220 seconds, depending on the prey's size.
It's not fast, but it's enough to make a noticeable difference when someone stays in there a little while.
Eating food and rubbing the belly doesn't change the vore digestion, but you'll also gain weight from the food you eat- the feeding and vore weight gain systems can both work at the same time.
Also to be clear, there's no concrete "end point" to the digestion- it just lasts until you decide to spit the prey out, or they decide to hop out of the chair. If you want to play with the sentient fat/ ghost hands system, the prey can just change into the ghost hands avatar and hop out whenever the two of you feel like it's been long enough.