Hmm.The short answer is "a lot". Do you mind if I now go on a bit? Feel free not to read the essay which will follow!
Dd's pony is kept at working livery at a yard 20 minutes drive away from us. There is public transport which she uses in the holidays, but in term time it takes too long. That means (skip this bit if you know) that she earns some of her keep by working in the riding school attached to the stables. Dd also does about 6 hours work a week in the stables to contribute. The good thing about this is that she gets exercised every day - the bad thing is that we have to trust the yard owner that she is being ridden properly. We do, mostly.
Dd has a lesson every Saturday morning. This means that someone has to drive her to the yard, then pick her up later. She tends to spend Sunday afternoon there - once again, take and pick up. She also goes to Pony Club on Thursday evening and rides on Tuesday evening. She sometimes works on a Wednesday. Her little brother often has to tag along on all this ferrying. Luckily there's a lovely pub opposite the stables with a big garden, so we quite often go there for lunch or a drink and a kick-about. He has also become worryingly good at pool!
On show weekends, she spends the whole of Saturday shampooing and primping the pony, then we all spend all day on Sunday at the show. This is crashingly boring with brief moments of excitement - it was entertaining watching the worst excesses of comptetitive "pony mothers", but that wore off after a while.
Apart from all this, we spend quite a lot of time talking about, thinking about, planning for and worrying about the pony. Or at least, listening while dd does.
But dd is exctatically happy, the pony is a much loved member of the family and we wouldn't have it any other way. But it does take over your life. Be prepared!