It can take any time between 10-30 minutes to become butter so it's better if you have lots of people willing to take turns shaking the jar. After a while the cream stops going "slosh slosh" and becomes silent. At this stage you have whipped cream. Keep shaking. Some time after that it suddenly goes "thud thud". Now you have a pat of butter in a puddle of buttermilk and you can stop shaking.
There's more work to be done. The buttermilk will turn your butter rancid quickly unless you can separate it all away from the butter. So pour the puddle of buttermilk into a cup and save it, but there is more buttermilk trapped inside the butter, like a sponge. Fill the jam jar with clean water, slosh it around, then pour the water away. Keep doing this until you have washed the butter clean. Now turn your pat of butter onto a wooden board and squeeze it with your hands to get the trapped buttermilk out. When you have pure butter you can stop.
You can use the buttermilk to make soda bread or scones and then spread the butter on them. If you have any home made jam as well, you're in for a feast.