My girls usually both sleep at midday each day, so once a month I'm able to sit down, have a coffee, watch an episode of something and work on my photobook spread for the previous month.
I use Big W photos online (just because I like that you can get free postage as you can get it sent to a local store), and I do a double page spread each month, from Dec - Nov (eg Dec 2016 - Nov 2017, so that I can get them printed well before Christmas, either to use as Christmas presents or to show family if we go away for Christmas).
I began when Miss M was born (2.5 years ago) as I was taking hundreds of photos each month and figured if I didn't do something with them, they'd end up just sitting on the computer!
I complete a two-page spread each month so that it's not overwhelming, and this means I am backing up all the photos on my phone each month as well. It probably takes me about 1-2 hours depending on distractions (and often I do it over 2 days - I upload the photos onto the computer and the website one day, and layout the pages the next).
The size book I use is roughly A4, and the amount of pages I use (2 for each month x 12 = 24 pages) costs about $40. I like to add in a blurb on each page of the highlights of that month. You can add pictures, text, embellishments and backgrounds as well - the possibilities are endless! I have kept it to a white background with black writing for the highlights section, and because I like to fit as many photos as I can on the 2-pages, there is no white space between the photos. As the kids get older and I either don't take as many photos or find I'm too busy, I plan to choose less photos each month - if you had a few kids you could even just choose 1 photo of each and 1 of all of them - and this means your layout would take considerably less time, but it means you are still documenting your child's growth and memories made each month.
I choose the best photos of the girls, but I also choose a range that show real life - messy faces, messy rooms, the tantrums, their angelic sleeping faces, selfies etc.
I love flicking through our past photo books; so far I have 2. Miss M also loves to see herself as 'Baby M' and loves to point out her Aunties, Uncles, Grandparents and friends.
If your kids are older and you wish you'd done something similar, start now! But instead of going back and documenting every single month, perhaps do a 10-page photobook for each year, with the best photos from that year. I guarantee you'll love the finished product!