The fastest upgrade would be a "standard" upgrade, which is NOT a live upgrade.
How fast that would occur depends on your infrastructure. A good rule of thumb is how long previous upgrades have taken (not live upgrade).
There are many factors that would affect whether an upgrade is successful or not from 8.6 to 8.8, like whether you have existing issues, specific configurations, etc... You should open a TAC case to see if anything you are doing would fundamentally block you from successfully upgrading.
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Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.
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