When setting up a search scope, the UI says:
Ready after next update (starts in XY minutes)
Is there any way to make SharePoint 2007 process the search scope at my command?
In Search Administration, within System Status, you can specify that the Scopes Update Schedule be set to "On Demand" rather than the default setting of "Automatically Scheduled Update".
Once a scope change is made, you can then kick off the update by selecting "Update Now" next to "Scopes Needing Update".
Alternatively, on an ad hoc basis, you could install Gary Lapointe's STSADM extensions, one of which is gl-updatesearchscope as in the example:
stsadm –o gl-updatesearchscope -url "http://intranet" -name "Search Scope 1" -groups "search dropdown, advanced search".
More details here.