Yield increase in sorghum has been achieved primarily by increasing grain number. Scope exists to increase yield by increasing grain size, however this has been limited by the negative correlation between grain size and grain number. Extending the duration of the grain filling period has potential to enable increased grain size without the trade-off with grain number. This study explored grain filling duration (GFD) in a diverse panel of 904 sorghum genotypes in three environments across two years. Significant variation in GFD observed, ranging from 400 to 680 degree-days, included entries with significantly longer GFD than current commercial hybrids. Longer GFD was shown to result in larger grain size. Additionally, only low associations between GFD and grain number per panicle, flowering time or plant height were observed, indicating that GFD could be manipulated without adverse penalty to these traits. A simulation study to estimate the benefit of an increased GFD across Australian sorghum growing environments over 60 years revealed positive impacts on yield when GFD was increased by either 10% or 20% in environments with low and mild post anthesis water stress but not in environments with sustained terminal water stress. However, maintaining overall crop duration by shortening time to flowering while extending GFD led to neutral or negative effects on yield. These results reveal opportunities to exploit GFD for improved genetic gains for yield in sorghum especially in environments or seasons where water does not become more limiting post anthesis.