I've had a Harpactira sp. drop a sac 12 days after mating and Yamia sp. Koh Samui after 2 weeks or so but this is not a common occurrence.
You could always just wait until you see your spider with a sac before getting containers as(depending on the species) you will have at least 6 - 8 weeks before there will be any slings that need potting up seperately. That said i always like to be prepared so i'd get the containers sorted as soon as i could.

What is the best practice, leaving the sac with mom or putting it in an incubator?

As Duke says, buy containers when you see the eggcase.

That is entirely down to personal preference. Some like to leave the sac with the mother full time, some like to take the sac at the earliest opportunity and others wait around 30 days then pull the sac. Just go with what ever option you feel comfortable with.

Doesn't the mother try to catch and eat her babies? If not, how do get the little ones out while you have a (protective?) mother in the same enclosure?

Down to personal choice.
I pull some of mine if they are nervous mothers or rare species that I won't be able to get a male for again.
But I have decided to start leaving the eggcases with the females for the breedings for this year and see how it goes. Mommy usually knows best.