from 1st hand experience i would agree with the 10min estimate of a NEW unsoiled garment (have had longer times with lighter rain).
i have revived my GoLite Wisp with McNett ReviveX DWR (sold on this website). i used 2x the amount recommended. not necessarily recommending that anyone do this, it's just that i 'm saying that i did this.
so...'pj', how did it work? you ask.
well, thus far i've had it out in moderate rain for short durations up to 15min. it has not wet through. it seems better than when new. keep in mind that these are totally uncontrolled testing conditions. heck...i'm not even really testing, so my impressions could be all wet.
i'd really like to know more about the pros and cons of using the silicone treatment a prev. poster mentioned. is it 'ok' to use it on the fabric itself - long term? i don't know. does anyone out there know?