The timer added is reduced based on the humours you have gained so far in the session.
After 1 humour level-up, the timer increases at ~0.74 of the rate shown on the food
After 2 level-ups, increases at ~0.62 rate
After 3 level-ups, ~0.56 rate
After 4 level-ups, ~0.52 rate
At that point I got bored, because I think I can already see what it is.. (the numbers above are approximate)
Let n be the number of levelups, let f be the full and fed up timer modifier
if n=0: f_0=1
if n>0: f_n=1-sum{[1/4]^((1+i)/2)} where the sum over i is from 1 to n
f_n = (2^n + 1)/2^(n+1) [closed form, pointed out by Tammer]
i.e. 1 levelup: f=1-1/4 = 0.75
2: f=1-1/4-(1/4)**1.5 = 0.625
3: f=1-1/4-(1/4)**1.5 -(1/4)**2 = 0.5625
4: f=1-1/4-(1/4)**1.5-(1/4)**2-(1/4)**2.5 = 0.53125
etc
Does this formula match other peoples tests? I don't know how many other people know this, so sorry if its really obvious.
NB. it asymptotes at f=0.5 as n--> infinity, and approaches that pretty quickly. So its not at all worth eating a food if its combined gluttony reduction modifier reaches a worse value than 0.5 (as part of a normal glutton… of course it may be worth it to just top off a levelup).