For delay times, here is a quick overview –
0-15/20 ms is not distinctly discernible and manifests as a timbre change (comb filtering kicks in early and begins to drive the timbre down). This area is good to play in to understand how the sound changes with adjustment.
20-50 ms is just discernible, and manifests as a “doubling” effect (this is where “chorusing effect” kicks in).
50-150 ms are easily discernible and sound as rapidly decreasing echoes (or “slapback echo”).
>150 ms delivers highly defined echoes (not often useful for voice, depending on application).
As far as feedback, also play with this… lower values are often best with voice. The delay is a filter loop that is introducing the original signal back onto itself modified by how you adjust the settings on the delay.http://forum.cakewalk.com/Vocal-delay-settings-m3007486.aspx