A target should spend a reasonable amount of time at each tier

text/0000-target-tier-policy.md

@@ -91,6 +91,15 @@ requirements.) A target proposal is encouraged to quote the corresponding
 requirements verbatim as part of explaining how the target meets those
 requirements.
 
+Note that a target must have already received approval for the next lower tier,
+and spent a reasonable amount of time at that tier, before making a proposal
+for promotion to the next higher tier. This policy leaves the precise
+interpretation of "reasonable amount of time" up to the approving teams; those
+teams may scale the amount of time required based on their confidence in the
+target and its demonstrated track record at its current tier. At a minimum, a
+target should normally go through multiple stable releases of Rust before
+seeking promotion to the next tier.
+
 In this policy, the words "must" and "must not" specify absolute requirements
 that a target must meet to qualify for a tier. The words "should" and "should
 not" specify requirements that apply in almost all cases, but for which the