Start with 5ms and see how it sounds. Anything above about 15ms is going to start being perceived as a slap-back delay and will start to defeat the purpose.
Just realize that this will wreak havoc on your mix when it comes to summing to mono and listening on anything with a center speaker channel. You might be able to find a nudge amount that'll both provide stereo widening and put the 2 waveforms in a reasonable phase with each other. I can't say that I've had too much luck with that.