A lot of FoF charts don’t have any time signature, that’s the issue. But time signatures are the same of any instrument: they dictate how many beats there are per measure and “how long” is a beat, which is true for any instrument.
I am well aware what a time signature is. What I’m saying is that a lot of FoF charters only chart guitar, so they time signatures are sometimes altered so that the guitar notes line up on measure lines. A common example is when the guitar part has some slight overlap either from one section to the next or after a solo. Many FoF charts will extend the measure to encompass all of the note. What happens then is that a crash or something on drums that denotes the start of a measure is a couple beats early.
I can probably dig up an FoF chart as an example if further explanation is needed.