If you and your partner are not legally married or registered partners, your relationship must be of a permanent and lasting nature. The Immigration Service will assess, based on all relevant information, if this is the case. Normally, you must be able to document that you have lived with your partner for at least 18 months at a shared address.
Such documentation includes rental agreements or registration papers showing joint tenancy, or documentation of extensive stays in each other's country of residence, such as plane tickets or copy of visa. In this case, you must also be able to verify that you have lived together while visiting each other. If will usually not be sufficient that you have lived together during short-term visits to each other's country.
Furthermore, your partner in Denmark must assume full responsibility for supporting you.
The requirement that unmarried couples have lived together for at least 18 months can be waived in special cases based on an individual assessment. Read more in this legal note about unmarried couples
from the Ministry of Integration (available in Danish only).