A demand letter is to be sent to remedy a breach of contract. I believe the request should be time-bound so that the defendant understands when expenses are will begin to accrue: expenses that they will be held liable (court filing fees, legal fees, etc.)
Is there a a rule of thumb for setting a response time? My initial reaction is 5 business days, because the plaintiff has emailed the remedy to the defendant and has been documented that a board meeting voted against remedying the breach.
I don't think there is a way to give a useful answer here. The time will depend on the nature of the alleged breech, the nature of the underlying contract, the industry, and how much urgency the demanding party feels the situation warrants. Some contracts will specify a time frame. If nothing is specified, probably a "reasonable time" is allowed. what is reasonable will depend on the customs of the industry involved, and the nature of the specific situation. If there is a safety issue, say or a risk of irreparable loss, things are more urgent than if it is say a purely financial issue.
In most contexts, a demand letter has no legal effect on the obligations of its recipient; it's just an informal attempt to resolve a dispute. So whether it demands a response within 15 minutes or 15 years, that's rarely going to have any impact on what expenses the recipient must pay.
And if the other party is represented by counsel, they probably already know when they will start to be exposed to the costs associated with litigation.
Otherwise, David Siegel's answer is mostly correct: Depending on the nature of the breach and the remedy being requested, I would typically expect a demand letter to offer the other side something like two weeks to respond. If the demand is very small or is time-sensitive, something shorter would probably make sense. But if the demand is asking for a huge sum of money, or if it makes nonmonetary demands that would take more time to evaluate and implement, something longer would probably be more reasonable.
