During the long outage at the University of Bern in Switzerland in 2005, a method was posted on how to extend the 14 day time limit.
The original instructions were as follows:

1. stop BOINC.
2. edit the client_state.xml file in your BOINC directory, taking a copy beforehand in case anything goes wrong. If the result name waiting to be uploaded is abcd_123456789 you need to search for that string in the file.
3. the search should find 6 occurrences inside <file_info> sections. Ignore the first one.
4. the other 5 should contain persistent file transfer blocks enclosed between <persistent_file_xfer> and </persistent_file_xfer> tags. Delete those tags and everything they enclose.
5. restart BOINC and you will now have another 2 weeks to complete the upload.

Note that the 6 in step 3, and the 5 in step 4, applied to the slab models, which was all that we had in 2005.
I'd suggest a full backup before trying this. :) Also, if you're at all unsure about it, make a copy of the file, paste it somewhere else, and practice on that. (But it's not too difficult, really. :) )

As it's an xml file that's being edited, care must be taken when saving, otherwise you'll end up with a text (.txt) file instead.
Not that it's too hard; just use Save. If you want the file to have a different name for some reason, change it BEFORE editing.

Also note that the number of times the persistent part occurs in the file will probably differ from one type of model to another, and also where you're up to in models with several phases.
I stopped all of my models before getting to a critical point, so I don't have any to look at and provide help on individual model types.

EDIT
For those that can't find the data folders, this may be because Windows has that area hidden by default.

To see it, go to the root of the drive where you allowed it to be installed, then go to Tools>Folder Options>View Tab. Select "Show hidden files and folders". It will now be visible.

If you don't know where to find it, this may help: The Big BOINC 6 Answer Thread