I tend to measure Splunk upgrade times in minutes rather than hours. Of course, that's the actual upgrade. The prep time is much longer and gets longer for each release behind you are.
Time is required to read each version of the Release Notes to see what effect that version will have on your environment. Splunk 8 introduces breaking changes for some apps that use Python so it's important to spend time reading the instructions and preparing for that upgrade.
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If this reply helps you, Karma would be appreciated.
