Day to day is pretty simple: I check all of my equipment/husbandry first thing in the morning. I also spot clean as needed and clean and refill water dishes every few days. I also check my feeding records daily to see which snakes (if any) are scheduled to eat that day. Some recommend feeding at night, especially with tricky eaters. I've been lucky so far I guess, all of my snakes will eat any time of day. So I have the flexibility of feeding when it works with my schedule. If I run into any trouble eaters I'll adjust accordingly. I also like to try to at least get a decent look at each animal every day, but most come out for at least a short period daily, with feeding end shedding exceptions as well as some days my schedule just doesn't allow much handling. I ALWAYS make time for daily check-ins and equipment/husbandry/water checks.
This almost looks like a lot, but it really isn't. With 4 snakes currently, it takes anywhere from 5-20 minutes for "regular" daily maintenance. The rest is time I choose to spend with the animals, plus a little time for feeding on the appropriate days.
I do a full enclosure cleaning every 4-6 weeks depending on the animal, the substrate, etc...
What are the chances of getting mites if your snake DOESN'T have them when you get him/her?
Not too sure on this one. Haven't had any mites yet. My understanding is they can come home with you on your clothes if you've been to a pet store or somewhere else with reptiles. I believe they can also come from substrate bags.