Show the math for the length of the refutation period (#411)

docs/architecture/smart-rollups.md

@@ -156,7 +156,10 @@ This kind of Smart Rollup is called a Smart Optimistic Rollup because the commit
 When there is only one commitment left, either because all nodes published identical commitments during the whole refutation period or because this commitment won the refutation games and eliminated all other commitments, then this correct commitment can be _cemented_ by a dedicated layer 1 operation and becomes final and unchangeable.
 The commitments for the next commitment period build on the last cemented commitment.
 
-The refutation period lasts 2 weeks on Mainnet; it can be different on other networks.
+The refutation period lasts for a set number of blocks based on the `smart_rollup_challenge_window_in_blocks` protocol constant.
+For example, currently the refutation period lasts 120,960 blocks on Mainnet.
+Mainnet has 10 seconds between blocks as of the Paris protocol upgrade, which means that the refutation period lasts 2 weeks.
+Ghostnet has 5 seconds between blocks but its refutation period is twice as many blocks, so its refutation period is also two weeks long.
 
 ### Triggering outbox messages