I expect something in the order of 50 GW at most (and 20 GW more realistically) until 30 June 2015. Last year, when installed capacity jumped from 3.6 to 4.7 GW, 2.15 TWh was fed back (and 2.25 TWh produced). Not an awfully high capacity factor, and indeed IEA-PVPS claims 1,000-1,1000 kWh per kW is "typical" (I guess clouds plax a role). Going with that, I would expect at most 55 TWh and more realistically 20 TWh annual production from what gets installed under the launch feed-in rates, that would be c. 5% resp. 1.8% of total production. For a significant price effect of a ¥42/kwh feed-in rate vs. retail prices in the ¥11.5-23.1/kwh range (from winter high-voltage industrial to large-consumption household, 2008 TEPCO figures quoted by IEA-PVPS) that negates the benefits via marginal pricing (which should be substantial due to the "peak-shaving" daily power curve), I think something well above 10% of annual production would be needed.

_{*Lunatic*, n.
One whose delusions are out of fashion.}