Just one child here and she's a young adult, so I obviously have a lot less life admin. than someone with multiple DC and/or medical issues necessitating regular GP or hospital appointments.
My bills are all paid via direct debit, so mostly it's annual stuff like renewing house/car insurance.
I do up a monthly budget on an Excel spreadsheet, to keep track of which bill is due out and when. I've been doing it for years, so it's second nature to me now and only takes about 15 minutes each month. I do a quick check throughout the month, to make sure I'm on track.
I need to keep an eye on my home heating oil and order more before it runs out. If it runs out, I'll get an air lock in the system which will necessitate a visit from an engineer.
The house requires occasional maintenance. For example, I need to ring a plumber because my kitchen tap is dripping atm. It probably needs to be replaced, so I'll need to pick up a new tap. If I run into problems with an appliance, I have to do some research and decide if it's more economical to get it repaired or replaced. If it needs to be repaired, I need to find a repair guy and take time off work. If it needs to be replaced, I need to do some research in order to buy a new one (size, cost, energy efficiency rating etc.). And book leave to take delivery of the appliance.
The car NCT (our equivalent of the MOT) is due every two years. So I usually arrange a pre-NCT service and then I have to book the car in for the NCT itself. Occasionally I have to book the car into the garage if a problem arises.
I meal plan every week and do up a shopping list. I buy a lot of the same stuff every week, so that's easy. But I have to remember to put the one off stuff on my list. For example, I need to buy a drain unblocker, because one of my sinks is very slow to drain atm. Yesterday I had to buy a new yard brush. Seems like every week there's a one off purchase.
I have two cats, so that adds to my life admin. Vet check ups, remembering to do flea treatments and administer medication or eye drops etc. One cat had to have surgery to remove some teeth (rescue cat).
Christmas involves a lot of life admin. Buying presents for assorted family members and friends. Arranging nights out with family and friends. Buying extra food and drink for the festive season. Making up beds if I have guests staying. I could opt out of Christmas, I guess. But I don't think that would be fair on DD. And it's an opportunity to catch up on friends I might not see the rest of the year.
Holidays require a fair bit of admin. Researching flights and accommodation. Booking airport transfers. Arranging travel insurance. Applying for an EHIC card if I'm travelling within the EU or EEA. Booking my car into the airport car park. Checking passport expiry dates and applying for a new one if required.
There's no NHS in Ireland. If I incur medical expenses, I've to claim back the tax relief from Revenue.
I'm separated and that involved a lot of life admin. Assembling assorted documents and sending them to my bank and solicitor. Re-sending them when they lost them. Endless phone calls to the bank and the solicitor to remind them I exist try to get things moving.
Which reminds me, I need to remind DD to apply for her college grant. I can't apply on her behalf because DD is the student. Except, if she forgets to apply, it will be me who will have to foot the bill for her college fees.
It's hard to quantify life admin. because it's not a regular, weekly task. A quick phone call might take two minutes, but I've often spent an hour or two researching holiday accommodation and flights. As a pp said, I spend more time thinking about the things I need to do than actually doing them 
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There was a lot more life admin. when DD was small, because I had to keep an eye on her school calendar (days off, non uniform days necessitating payment, dress up days etc.) and her clubs (payment dates, dance class rehearsals and shows, Scouts events and days out etc.). Buying school books and uniforms for school and clubs.