Probably a lot more than your ceiling can take. See FAQ: Will my loft floor be strong enough for flooring?
StoreFloor is extremely strong and LoftZone is the only raised loft floor manufacturer to have put its products through robust accelerated lifetime tests in extremes of temperature beyond those normally found in the UK. Even in these conditions, a StoreFloor deck survived loads of over 500kg per square metre, without failure.
In addition to being strong itself, StoreFloor’s lattice-beam structure also strengthens the joists it is screwed into. Nonetheless, StoreFloor is only intended for storage and occasional access, and we recommend that you don’t overload your joists. If in doubt, consult a structural engineer.