It's hard to say as you can get pretty early random shinies during
a chain, but in general you can kind of expect a shiny somewhere
between chain 60-80. But it's really luck based. I'm currently
hunting Oddish with Premium (2,75% shiny chance) and have hatched
101 Pokemon without a shiny. My previous hunt (Raylong) gave me a
shiny at 42. So yeah, 60-80 for the first shiny is pretty average
and then a shiny roughly every 50 Pokemon
(from my
experience)
I just saw your chain and yes, even without premium, 150 hatched
Pokemon without a shiny is considered very unlucky :) Sorry
In case of emergency Plushie