Impossible to answer I'm afraid. It just depends, some would charge set weekly/monthly fee, others would charge 50% of costs etc.....everyone is different. I've got new starters in January and took deposits from them all as I don't actually have the space until January. If I had a space now and was reserving it for then, I would HAVE to charge a retainer but I'm not sure what as I've never done that before.
Same as Shoshable-i charged from April-August at just less than 50% of the weekly rate of 3 days, as the parent was very happy with me and i had the space there to fill, so this secured it and meant the Mum had a very relaxed few months before her return to work.