Stacking Bonus Rule Question

Does "Each attacking creature you control gets +1/+0 " stack? My example is from this Tuesday, Andrew had the Gruul War Chant out
and swung with three creatures.
The next round Andrew attacked with the same three creatures. The question is did the +1/+0 from the previous round fall off or are the creatures +2/+0 this round when they attack.
We ruled that the +1/+0 fell off after combat completed and that they were +1/+0.