Now this is an interesting question. I've been mixing rules for so long now I had to go and look up the originals.
In Mayday one sand canister produces a cloud of sand that proveides a DM of -3 for incoming laser fire and -1 on missile attacks. Outgoing laser fire receives a DM of -1. Note, the sand cloud does not dissipate when hit by laser fire, only by the ship changing vector.
In LBB2 one canister provides a DM of -3 to incoming laser fire per 25mm/0.5"/range band (depending on scale/rules edition) of sand, implying the sancaster produces a cloud 25mm/0.5"/range band thick per canister launched. The rules on ordnance imply the sand canister may be detonated away from the ship in order to get extra thick sandclouds, Again, there is no mention of them dispersing. And also note ther is no mention of damage to missiles.
In SS3:Missiles, missiles are incapacitated if they fly into a sandcloud and roll 12 on 2d, roll once for each 25mm of sand they fly through.
Hope that helps.