i think it will be a LONG LONG time. i drove from DC to Philly this weekend (3-ish hour trip one way) and only saw 1 Prius on the way up, and none on the way back down. there just aren't enough on the roads. maybe someday there will be.
the only way i can imagine two Prius colliding is if the drivers are both waving to each other (not paying attention to driving), both in glee that they have their car AND finally someone else does too!
One hour after posting that idle question I saw my first other Prius on the local streets, black and sleek, passing through an intersection I take to work. I feel that we are destined to meet one day... *shudder*
the first time i saw one in my neighborhood, i shuttered too! the lady driving it saw me, i saw her, and we both beamed with the face of two proud Prius parents, and waved happily at each other (hence, my guestimation of the first possibly Prius/Prius accident).
Here in the south of UK the chance of a Prius/Prius collision must be infinitesimal. I've had mine for 8 weeks and not seen another one from Bristol to Brighton. I expect it's a different picture in London though. Since Prius owners don't have to pay the 5 pounds congestion charge to drive in central London they must have to spend all their time dodging the jealous drivers of other models. When they meet another Prius they might forget to dodge!