Fuji Abound,
Cloud flying in gliders is comparatively rare. Most of the time a glider makes better progress by staying below cloud base. It tends to be used for three purposes: to cross large gaps where there is no lift (e.g. where a heavy shower has temporarily killed off thermic activity) or possibly to climb over the top of isolated clumps of airspace like Mildenhall/Lakenheath, (more likely) to get home at the end of an XC flight when the cumulus are far apart, or to let down through cloud after climbing in wave. On a five hour flight I'd expect to spend no more than 15 mins or so in cloud, and that on maybe one flight in ten. I think this is pretty typical, but those who actually do fly cloud may be able to add to my examples.
On a good soaring day I'd predict fewer than 100 gliders taking cloud climbs over the whole of the UK, so maybe 1500 minutes of cloud flying in total, but this is based on no more than gut feeling. If there's good, regularly spaced cumulus I'd be surprised if anyone were cloud flying except to practice.