Someone was asking for mass-flow numbers earlier on. RR's Web site gives an intake mass-flow for the RB211-535E4B (I picked an engine arbitrarily) of 1,177lb/sec or 533.87kg/sec.
Based on the figure of 0.3 milligrams per m3 given for Stranraer, at an air density of 1.2kg/m3 at sea level (obviously we're not interested in sea level, but at least it's wrong in a known way - the ash measurement is a sea-level one and I guess RR's figures are test-stand measurements, so it's consistent) that would be 444m3 of air a second and 0.133g of ash a second - 478g of ash per engine-hour.
(Although, the -535 is a very high bypass turbofan, so perhaps we need the core mass flow...)
Now, scientists and engineers have agreed a threshold concentration for ash of 0.002g per cubic metre of air. At or below this concentration, there is no damage to the engine.
BBC News - Was the flight ban necessary?
Thats a pretty high level.