One of the "big deals" in FreeBSD these days is ZFS. The issue is that ZFS is under a non-compatible license and has patents preventing clean-room re-implementation.
My question is for how much longer will these patents be valid?
ZFS is a big deal not only on FreeBSD but also on Linux. It is a fine file system and the issue has been as far as I know as much about the patents that about the license of ZFS itself:its license is not compatible with the GPL which is problematic in the context of a Linux Kernel file system driver:
[...]difficulties arise for Linux distribution maintainers wishing to provide native support for ZFS in their products due to potential legal incompatibilities between the CDDL license used by the ZFS code, and the GPL license used by the Linux kernel. To enable ZFS support within Linux, a loadable kernel module containing the CDDL-licensed ZFS code must be compiled and loaded into the kernel. [...]
Patents are threats to any software endeavour. In the case of ZFS there are issues at multiple levels:
a significant number of patents from Oracle/SUN that would need extensive reviews to understand their scope and life span.
possible challenges of these patents by other patent holders with secret settlements and unclear impact on the FOSS status of ZFS:
[...] NetApp and Oracle have agreed to dismiss their respective lawsuits against each other without prejudice. ZFS-using companies such as Coraid and Nexenta can now go ahead free of the threat of NetApp interference. The dismissal terms between the two companies are confidential.[...]
So this is not an answer to your question proper. But answering it would require a significant amount of work that cannot be accomplished here.