SLO Measurement Window: 30 Days vs 7 vs 90
The window choice changes the SLO’s personality. Pick to match your business cadence and on-call appetite.
What the window actually controls
The measurement window decides "how much budget you have at any moment." A 30-day window with a 0.1% budget equals 43 minutes of bad per month. Shorter windows mean less budget and faster reaction; longer windows mean more budget and slower reaction.
- Window plus target equals budget. The two parameters together determine "how many minutes can be bad before we burn the budget."
- Shorter window. Less absolute budget; the team reacts to small regressions; on-call gets noisier in exchange for faster signal.
- Longer window. More absolute budget; the team reacts only to sustained regressions; calmer on-call, slower feedback.
- Pick deliberately. The window is a product decision, not a default; it sets the SLO’s personality.
Four common windows
Four window choices cover most SLOs. Each fits a different action cadence; pick the one that matches when the team can actually respond, not the one that matches the calendar.
- 7-day. Sensitive; tight feedback loop; right for sprint-cadence teams that want fast signal.
- 30-day. Standard; matches monthly business reporting; the safe default for most services.
- 90-day. Generous; long-tail trends visible; right for stable services where small regressions are noise.
- Annual. Contract-aligned; matches SLA windows; rare for engineering use because action cadence is too slow.
Rolling vs fixed
Rolling: window slides; budget refreshes daily.
Fixed: window resets at month/quarter end; budget refills cliff-style.
Rolling avoids cliff drama; fixed aligns to billing.
Picking by business cadence
Match window to action cadence. If action is monthly, use 30-day rolling. If action is sprint-based, 14-day might fit.
The window is a tunable; revisit annually.
Antipatterns
- 7-day window for noisy services. Always burning.
- Annual window for engineering. No actionability mid-year.
- Mixed windows in same dashboard. Confusing.
What to do this week
Three moves. (1) Apply the pattern to your most-impactful service. (2) Measure adherence for 30 days. (3) Rewrite the policy or the SLO if the gap is durable.