I used to work for a family where i had every morning off (except school holidays / if children sick)
The "deal" was... so long as food is prepared / childrens laundry done / and bedrooms / playroom had a sort out once a week or so then other time was mine.
I used to go in all morning on a Monday.... and get on with the washing. (they had an industrial sized washing machine, and mummy liked the machine to be full so i normally got all the clothes / bedding / towels in this one super huge machine!)
Would tidy the bedroom and play room (they were never that untidy) (Mummy and Daddy (and me!) agreed it was mad to tidy everything away every night, especially things like playmobile / polly pocket /sylvanians that would take ages to set up ready to play. However the rule was when the cleaners came once a week everything had to be away so as not to get hoovered up!
Id typically cook a couple of meals.... Monday / Tuesday. and sort out a lunch too. (something more than a sandwich! perhaps cauliflower cheese, jacket pots in oven on times, with a salad prepared)
Then Tuesday id go in about an hour before school pick up and iron / put away the laundry from the day before.
Wed was swimming, so id go in and make a meal that could go in the oven on timer, as we wouldnt get home until gone 5pm (and be starving!) and children normally started dinner at 4.45.
Thu / Fri id normally have the whole morning off, and would make a more snacky quick lunch. ( a sandwich / reheat some soup etc) when i arrived (with children) after lunch.
However my phone was on and i was always available to get children if the school called for whatever reason etc.
It worked for us...... I always had a freezer stocked with portions of home made food (that parents could use at weekend, though normally didnt,) and in the holidays id run the freezer down etc. Also MB was vegetarian, so when i cooked something meaty id always make enough for DB to have some for his tea too!
So long as laundry sorted / fresh food was prepared / and childrens areas tidy and organised MB / DB didnt mind how i organised my time to fit it in. 
If DB was away in buisness id happily walk the dogs, and I also sometimes took dogs to dog groomers on way to school, and MB would collect dogs on her way home from work etc.
Id also house / dog sit (for free, but i LOVED it as the house was amazing, and i could help myself to food, and have friends round, swim in the pool etc!) if they went away for a weekend, and babysat sometimes, so sometimes id time the washing so i could iron while babysitting / dog sitting, and then have a little more time off on the monday morning!
again so long as it was done MB / DB really didnt mind when!