It is well-known that some human contact including petting and touching is necessary to socialize kittens to be comfortable around humans. I have no experience with this and work a day job, what is the typical amount of time I need to spend with them together or one-on-one from birth until 8 weeks+ to help prepare them for living with people?
E.B. Karsh did a study on the effects of human contact on kitten behaviour published in 1983. Some essential, and interesting, learnings from that include:
Kittens handled only 15 minutes a day from birth through to 12 to 15 weeks would accept human affection, but would tend to wander and return rather than staying in place.
Kittens handled for an hour or two for the same period would be sufficiently relaxed to curl up on people and sleep.
R.R. Collard, in 1967, noted that more handlers tended to make kittens more comfortable around humans.
There are a number of additional studies that show very early and regular handling have a positive effect on kitten development, so the upshot of it all is handle them frequently and for long durations right from the start all they way through their development.