What is longest Cat 6 cable I can run when I am just using it to connect a single user to a switch?

The UTP standard, to which the various ethernet standards adhere, is 100 meters, but that assumes 90 meters is solid-core (better performance, more fragile), with 5 meters on each end as stranded (worse performance, more flexible).

It has nothing to do with daisy-chaining, but with several measurements, such as frequency, insertion loss, NEXT, PSNEXT, FEXT, ELFEXT, PSELFEXT, return loss, propagation delay, delay skew, balance, longitudinal conversion transfer loss, etc.

The 100 meters also assumes that the cable is pre-built or professionally installed with all the same rated components, and tested to pass the full test suite. Even experienced installers have problems when installing Category-6 cabling, although Category-5E cabling can do 1000BASE-T at the same distance. Category-6 cabling can do 10GBASE-T at 55 meters, but Category-6A can do 10GBASE-T at the full 100 meters.

If you want to use the common copper 10, 100, or 1000 Mb/s Ethernet, 100 meters is the standard; any longer and nothing is guaranteed to work right.

However, as far as the longest cat6 cable between a computer and a switch, if you add something like a pair of G.hn bridges you can then use 2700 feet of cat6 between the two bridges, and enjoy one full megabit of bandwidth.