If it's badly furred up then it'll take a few hundred miles. Initially it might drive strangely as the baked on carbon will get hotter (this is a good thing, means it's burning off) but will lead the ECU into fiddling with fuelling to keep the oxygen sensor happy. This is good. Excess air will help burn away the carbon deposits. Give it grace for this tank fill, then do the same again.
There's a lot to be said for the old fashioned Turin-Tune-Up technique. It involves letting the enigne warm up gently and then checking the engine oil level after it's been parked for an hour or so. Top up to the dipstick max.
Now, restart and let it come to operating temperature whilst driving half sensibly. Find a good clear section of dual carriageway and drive at the legal limit checking for mobile speed traps.
Go right round the next roundabout, back the way from whence you've come, and boot it. All revs in all gears. Next roundabout, round again and off you go...again! A couple of dozen miles of this will remove the remaining dirt and clean the exhaust and cat too. Let it cool before turning off (yes, even nat asp engines need time for oil to cool to avoid baking deposits onto piston rings etc.)
Next fuel visit, refill tank with decent unleaded (no TEsco rubbish) and a dose of Redex then go and enjoy a clean and lively engine.
It's a good way to prep your car for a MoT emissions test.
PS - I like the 8v 2.0 Golf far more than I like the the 16v. It's torquier at low revs and so much much quieter.
RGds,
P.