GXY should be more clear on its terms. A reserve is what's feasible to mine, a resource is what's in the ground but may not be economically feasible to mine.
Assuming the ore RESERVE is 15Mt (as per their recent announcement) that would indicate production of 1.5Mt of spod/Li concentrate (concentrate produced looks like 10% of ore).
1,500t/200t per year = 7.5 years.
GXY's CY17 forecast is 160t (lower due to ramp-up), so CY18 should definitely achieve 200t.