With tomatoes, I let them develop two sets of adult leaves, (the first set of leaves are ‘seed leaves’) and get about 6” tall, then I re-pot them DEEP into a tall 4” pot. They will root all along their hairy stem, making for a stronger plant.
Pretty much rule of thumb for my peppers too, but DO NOT plant them deeper than they already are in the starter pot/plug.
Everything else, I just eyeball. I try to time things so that they are no more than 8 weeks old from my last frost date in Spring and then out they go into the garden beds.
There are exceptions, of course.