couple of weeks - photocopy the original, they are good at losing things....................
It wasn't so easy for me, I changed from a 3.5V8 that died to a 3.9 V8 (much easier to get) and the DVLA then wanted an official receipt from a garage for the engine, or a garage report on the engine number and cc, or a VOSA inspection. The DVLA will not just accept a change in CC of the engine..