The deadline for appealing to the Immigration Appeals Board (Udlændingenævnet) against a decision from the Danish Immigration Service (Udlændingestyrelsen) or SIRI is normally 8 weeks from the day you were notified of it. Treat that date as fixed. An appeal that arrives late is usually rejected without being considered at all.
The question that follows almost immediately is what happens to the departure deadline stated in the refusal. An appeal can carry suspensive effect, which means you are allowed to remain in Denmark while the case is being processed even though that deadline is approaching. Whether it does depends on the type of case and on the rules the decision was made under.
EU cases follow their own rule. If the refusal was issued under EU law and you appeal within 7 days of the date of the decision, the appeal will generally have automatic suspensive effect, and the departure deadline is put on hold until the Board has dealt with the case.
That does not carry over to refusals under the Danish national rules on residence and family reunification. There, suspensive effect is decided case by case, and there is no automatic right to stay in the country while the appeal runs.
Because both deadlines are short and missing them is hard to undo, it is worth having the refusal read quickly. Beyond filing on time, there is often a separate argument to be made for why you should be allowed to remain in Denmark while the case is decided.