Aim is three sessions a week. 45 mins to an hour. Medium to high intensity, giving sufficient time for recovery. Then once I plateau with that look to change it.
Luckily the youngest is soothed by the white noise....
You can do recovery or endurance sessions in between the proper hard ones, I find it better than doing nothing. Or just take it easy in the commute, it's good to flush abused legs
My fitness nosedived catestrophically while my 3 year old was younger, so even more impressive.