Anywhere from 2-10 hours a night. It depends on how much is going on. Sometimes it is tricky to switch off, and at other times you're so tired/exhausted, you're asleep in seconds. I find that during winter I need more sleep. Where I live, cheeping birds naturally wake me up, as my garden is right outside my bedroom.
I aim for at least eight, but ever since I became a mother about 8 years ago, I don't remember a night where I got the eight hours in a single chunk. I wake up at least 2-3 times for no good reason. I miss sleeping! :)
On average I'd say 7.5 hours a night although if it's been a really busy day sometimes I go to bed a bit later than usual. Other nights I'm really tired and have to hit the sack early. I work full time at a school and I must say by the time the school holidays arrive I am exhausted and dragging myself out of bed in the mornings. Definitely not enough hours in the day when you are a working mum!