The "global scan time" setting (in system settings: "Scan interval") appears to only take effect when creating new subnets, not existing ones. The only way to "change them all" appears to be to go into the database and hand edit/select fields in multiple tables. While a its nice to have a 'workaround", this really should be an option so you can change "all the subnet scan times" at once, or better "update any that have not been locally set/changed". Hand selecting subnets (as some have suggested) is do-able, sure (and selecting multiple subnets at once does not appear to allow changing the scan interval setting) but being able to update the settings for all globally, from one configuration would be very nice, especially when the system grows and the scanning jobs begin to back up and no specific subnet is the cause.
Here are some posts discussing this: IPAM Subnet Scan Frequency Large Change - Forum - IP Address Manager (IPAM) - THWACK (solarwinds.com)