I set this value to 999 seconds and searched for 999 in /registrymodifications.xcu, which is the file where most settings are stored. The value could not be found, so I had a second look at the options dialog and found out that LO had set back this value to 600 which seems to be an upper limit.
So I tried 555 and found 555 at
<item oor:path="/org.openoffice.Office.DataAccess/ConnectionPool/DriverSettings/org.openoffice.Office.DataAccess:DriverPooling['com.sun.star.comp.sdbc.calc.ODriver']">
<prop oor:name="Timeout" oor:op="fuse">
<value>555</value>
</prop>
</item>
I copy the whole path /org.openoffice.Office.DataAccess/ConnectionPool/DriverSettings/org.openoffice.Office.DataAccess:DriverPooling['com.sun.star.comp.sdbc.calc.ODriver']
, start MRI, call mri-menu:Targets>Configuration… and paste the path into the prompt.
Now I see all the UNO properties related to that configuration node. Among them:
DriverName=com.sun.star.comp.sdbc.calc.ODriver
Enable=True
Timeout=555
I turn the MRI tool from get mode into set mode, double-click the Timeout property and — nothing happens.
Having a closer look at the property:
Timeout long 555 Maybevoid,Bound,Constrained,Read_Only
At this point I’m stuck for now.
P.S. 555 and its config path could also be found and modified in Tools>Options>Advanced [Open Expert Configuration…]