As far as I can tell, transfiguration mana requirements are based on the size of the assumed form, and there can be a huge size change (such as a 20 kg? rocket from a small ice cube taking a long time) and the very easy partial-transfiguration slicing.
Harry doesn’t need to keep the rock at all times. He just needs to contact it more often than once every transfiguration duration.
As far as I can tell, transfiguration mana requirements are based on the size of the assumed form, and there can be a huge size change (such as a 20 kg? rocket from a small ice cube taking a long time) and the very easy partial-transfiguration slicing.
Harry doesn’t need to keep the rock at all times. He just needs to contact it more often than once every transfiguration duration.