If you return control to the user under 150ms your app will fell quite snappy. From 150ms to 200ms is okay too. Anything above 250ms is noticeable and considered by many as sluggish.
Another rule of thumb that's used is that responses within a tenth of a second feel instant, within a second will keep the user focussed, and you can get up to about ten seconds before they'll want to do something else while they wait.
