Maximum length of Atmel AVR ISP ribbon cable?
There is no formal maximum length cable (within sensible reason) and the maximum reliable length varies with the quality of the implementation.

As the cable gets longer the capacitance and inter conductor noise coupling rises* and depending on design decisions (or lack of decisions) by the makers, maximum reliable length will vary.
There are some designs which may be "flaky" at 1" cable length and I'd start to get nervous using an eg 10 foot cable with even the best of designs (but it MAY work OK). But, 1 or 2 feet should be OK in most cases. Erring on the shortest sensible length will do you no harm electrically.

Trying 2 feet and seeing what happens costs you an extra termination at one end (or a reused plug if you are clever and lucky) if you have to shorten the cable due to reliability issues. 



*Inductance and resistance increase as well but I'd generally expect capacitive coupling and loading to be the key issue.