Currently long runs have a pretty short deadline compared to the time, which it takes to process them causing many of them to fail to get processed in time due to computer not being on/computing all the time. Any possibility to extend that deadline, say to 10 days or so? Pretty frustrating to notice that computer has spent numerous hours crunching for nothing.

This has been discussed before and I don't think they'll extend it.
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Team Belgium

Turn down your minimum and maximum additional work buffer to something like 0.05 or 0.1 days. That will keep BOINC from downloading a new work unit until the current one is almost finished.

They will not extend the deadlines, since each work unit, when completed, forms the beginning of the next work units (in a nutshell; it is a bit more complicated :P). They must come in soon after they are sent out so the queue can stay full and not waiting on work units that have been hoarded (either on purpose or by accident).
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XtremeSystems.org - #1 Team in GPUGrid

You have 2 choices:

Subscribe to the short queue or setup a script that will wait until the GPUgrid task completes, uploads and reports before shutting down your PC.

Also, if other projects are pre-empting your GPUgrid task then you should set aside 1 core. That way the GPUgrid task will never get pre-empted and it will finish sooner.

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BOINC <<--- credit whores, pedants, alien hunters