Matt feels like he could do this for a long, long time.

All of it-- not just this part, the sucking off part, but the spell and the touching-- God, he could touch him for hours.

His mouth starts to move faster, lips wrapping tight, and he stays mindful of Sherlock's breath and the sounds he makes and the level of cold.

The spell's been great so far, but he didn't fine-tune it to accommodate swallowing.