Easy to calculate, the breeding ratio of light water pressurised reactors is around 0.6-0.8, so each TWyear electrical output will make around 600-800kg Pu.Plutonium enrichment by centrifugation is generally considered infeasible because of the mass difference between isotopes - 1 for Pu 240/239 vs. 3 for U 238/235 - but there's an interesting laser enrichment technology that utilizes the difference in magnetic properties of the nuclei, not the mass difference, to preferentially ionize one isotope which can then be captured by an electric field. All the work I've seen on it uses uranium enrichment as an example, but it might be feasible to use it to enrich an existing commercial grade Pu stockpile.
The 2.9+- 0.6 figure I've seen for China's plutonium stockpile is the weapons grade stuff, I wonder how much commercial grade plutonium it has.
Of course later on the reactor will burn that as well, but this can give a magnitude estimate about the amount of available commercial grade Pu stockpile.