According to Matt Simmons, a retired Aramco executive said that Ghawar would never make more than 70 Gb (41% recovery).  Matt also said that the world record recovery factor for this type of reservoir was 45%.   I think that Ghawar is getting close to 60 Gb (35% of OOIP).  

Note that depletion starts when the first barrel is produced and it never stops until the last barrel is produced.  During this time period, a field can show rising production, stable production or falling production (some rate of decline), but regardless of whether production is increasing, flat or declining, depletion marches on.

Note that Shell was expanding their surface facilities at Yibal (same reservoir as Ghawar, also redeveloped with horizontal wells , just like Ghawar) to handle a projected increase in production, when the field started declining sharply, as the water hit the horizontal wells.   If memory serves, the final decline at Yibal hit when the field had produced about 35% of OOIP.