while i do prefer longer games, i say it all depends on the kind of game it is for the length.
like say we got space thumper and untitled farming game, both from the same person.
for space thumper, the weight gain is the primary mechanic and it really isnt slow. so its more suited for shorter under 2 hour sessions.
for untitled farming game, the primary mechanic is farming while fattening things is a reward for that. along with the fact that the weight gain is a bit slower. its more suited for longer sessions.
there is also the fact of how long it takes to get to the fetish content, which also depends on the kind of game it is. for shorter games you of course expect to get to the fetish content right out the gate, while with longer games id say 15-30 minutes is a respectable amount of time to get to the fetish content, as long as there are not gaps that big between, and there is a genuine reason for the delay. such as if its a factory building game with WG content. you of course are going to need to get initial automation set up before you can do anything anyways, which should be reflected in the fetish content. but if its something like the witch of gluttony, then the weight gain should happen right after the intro sequence.
so overall:
shorter games:
should follow a system where weight gain is/is part of the main mechanic and you get to the fetish content right out the gate. as spending most of the game working for the WG only to have a few minutes of it is quite a buzzkill
longer games:
should follow a system where weight gain is the reward for the main mechanic (while it is fine if its given its own mechanics), and a small time period before you can get to the weight gain content is acceptable. just as long as there are not large periods between WG content. there should be a near constant stream of fetish content immersion once it starts, in my opinion.