The maximum length is determined by many factors.
Firstly, the interconnect - you want to run signal/gnd/signal/gnd on the cables if using flat cable or use twisted pairs.

Then there is the clock speed, obviously the length is less with the higher the speed.

Also there is the type of drivers and receivers used. Standard cmos in/out of the ics are really only good for short distances (<30cm). Using RS485 type drivers extend the distance quite significantly.

SPI and I2C were really only meant for short on board or board to board interconnects. The run an extended distance requires a little more consideration.