Downtime Budget Planning: Allocating Maintenance Windows Within SLA

Planning Downtime Within Your SLA

Every SLA has a finite downtime budget. Smart operations teams proactively allocate portions of this budget for planned maintenance, deployments, and infrastructure changes, leaving a reserve for unexpected incidents.

The 70/30 Rule

A common practice is to reserve no more than 70% of your error budget for planned activities, keeping 30% as a buffer for unexpected incidents:

SLA	Monthly Budget	Planned (70%)	Reserve (30%)
99%	438 min	306.6 min	131.4 min
99.9%	43.8 min	30.7 min	13.1 min
99.95%	21.9 min	15.3 min	6.6 min
99.99%	4.38 min	3.1 min	1.3 min

Maintenance Window Sizing

Based on your SLA and the 70/30 rule, here is how often you can schedule maintenance:

At 99.9% SLA (30.7 min planned/month):

• Weekly 7-minute maintenance window ✅ (28 min/month)
• Bi-weekly 15-minute maintenance window ✅ (30 min/month)
• Weekly 10-minute maintenance window ❌ (40 min/month — exceeds budget)

At 99.99% SLA (3.1 min planned/month):

• Monthly 3-minute rolling restart ✅ (barely fits)
• Weekly maintenance of any duration ❌ (not feasible)
• Must use zero-downtime deployments ✅

Deployment Frequency Impact

Your deployment strategy directly affects your downtime budget:

Strategy	Typical Downtime per Deploy	Max Deploys at 99.9%
Full restart	2-5 minutes	6-15 per month
Rolling update	10-30 seconds	60-180 per month
Blue-green	1-5 seconds	360+ per month
Canary/progressive	0 seconds	Unlimited

Sprint-Based Planning

For teams using two-week sprints:

Sprint downtime budget = Monthly budget / 2

At 99.9% SLA:
  Sprint total budget: 21.9 minutes
  Sprint planned budget (70%): 15.3 minutes
  Sprint reserve: 6.6 minutes

This means each sprint can include planned work that causes up to ~15 minutes of downtime.