The implication is that Stormtroopers are responsible for the superficial cleaning of their own gear. They certainly clean their own weapons, at least within their barracks, with teams of 'techs' taking care of any actual equipment malfunctions.
Finn knew the layout of the dark flood tunnels like he knew how to
clean his rifle. He could travel through the maze with a blast shield
over his eyes and still get where he wanted to be.
Star Wars: The Force Awakens - Official Junior Novelisation
It's noted in the 'Official Star Wars Fact Files' that the armor itself is incredibly durable and more than capable of lasting for the lifetime of the soldier. Maintenance shouldn't be much more than simply wiping it clean with a damp and dry rag
"Combined with a helmet, this armour could be made airtight, providing
limited protection in a vacuum. It was also durable enough to last
Star Wars Fact File:Vol.2, #13
Within the EU, we see clone soldiers cleaning their armour between missions.
Fi glanced into some of the rooms to check who might be awake and willing to chat, but the Delta boys were all out cold, not even snoring. Niner and Corr slumped in chairs in one of the alcoves with a plate of half-eaten cookies sitting on the small table between them. Darman was stretched out on his bed in the room he shared with Fi, apparently none the worse for his ordeal, and Ordo was curled up in the next room with a blanket pulled over his head. Odd: he always seemed to do that, as if he wanted total darkness.
There was no sign of Jusik or Etain. Farther along the passage, Fi struck lucky. Atin was sitting in the chair in his room, cleaning his armor.
Republic Commando: Triple Zero
“Not since yesterday.” Niner cleaned his armor. It didn’t matter how
visible they were now, and he hated scruffy rig. Darman had stripped
down his Deece and was wiping the ignition chamber more than it needed
it. Fi wandered around the temporary camp, cradling his rifle, keeping
Republic Commando: Hard Contact