It's really up to each issuers policy in which not only the circumstances surrounding the dispute, but also the classification of the dispute (fraud, services or product never delivered, etc), influence, or determine the length of time they'll issue a charge back allowing the ensuing rebuttal (if there is any)
For instance, if it were a dispute due to fraud, or a stolen card it would likely be a much shorter window (say 60 days) in which they'd allow a dispute. This prevents people from claiming all of a sudden, say 6 months down the road, that the charges are fraudulent.
In contrast, lets say you ordered a custom made widget from a widget maker, you paid in full, waited what anyone would consider more than a reasonable amount of time for said custom made widget, but, they never delivered it, you asked for a refund, they refused the refund because it was made to order.
In this scenario, I've personally seen Amex accept a dispute and subsequently issue a charge back 18 months after the purchase was made. In this case the customer had signed a purchase agreement acknowledging it was custom made, with no promise of delivery date, but most importantly, they were not entitled to any refunds once production had begun. They lost the dispute and the money was returned to the widget maker (or merchant). Had there not been that signed agreement (in some cases even a signed CC receipt) the dispute would have stood and the charge back made permanent.
