Why FT8 uses fixed, clock-synchronized transmit/receive slots of 15 seconds, starting at 0, 15, 30 and 45 seconds past each minute. The actual signal lasts about 12.6 seconds within that window, leaving time for decoding and for the operator to pick the next message. Because the slots are fixed, your computer clock must be accurate to within about a second or you will not be decoded.
Watch out The choice saying it varies with the amount of data describes packet or other asynchronous modes; FT8 always sends the same 77-bit payload in the same fixed-length frame. The 30 second and 8 second options confuse it with other WSJT-X modes: JT65 and JT9 use 60 second cycles and FT4 uses 7.5 seconds.
FT8: the 8 is the 8-FSK tones, not the timing. Four slots per minute means 15 seconds each.