well every 3-5k depending on oil and driving style
regardless, one oil change per year is horrible.

as for engine options, a complete ver 7 STi swap is plug and play with and 02-03 and you have about 8 wires to run to get AVCS working...aside from that, a usdm STi swap can work but AVCS won't unless you get a custom Hydra (the cost isn't too much more) to run the AVCS...or you can just toss in another none avcs turbo motor (2.0 or 2.5) and get a reflash done to correct your ecu for fuel and timing (only needed if going 2.5)

regardless you probably toasted your entire motor (heads too because of cam journals) and should be looking to do a complete swap/build.