And can states be held accountable for any violations of the treaty that occurred before they incorporated the treaty into domestic law ?
Treaties are obligations to the world. Domestic law creates obligations within the state's own sovereign domain. A country assumes the obligations under the treaty when it signs or ratifies it (which of these is necessary depends on the treaty) and it comes into force (the trigger for this also depends on the treaty).
A country's self-perception/self-description as "dualist" only means that it does not view its domestic law to be affected at all by international treaties. Treaties only have effect domestically through positive enactments by the legislature. The dualist view does not assert that the obligations do not exist at international law.
I also note that even for dualist countries, many aspects of treaties speak only to international obligations, rather than to anything that must be done internally. For example the obligation to not destroy cultural heritage during a war is an obligation at international law regardless of whether a nation has enacted positive law directed at its military members to not do that.
While a country's internal implementation process may be slow, this will be noted by international UN oversight bodies, in their compliance reports.
Some sovereign nations do not even have sole capacity within their own domestic law to fully implement a treaty. For example, Canada's federalist division of powers places some matters within the exclusive jurisdiction of provinces. So, even though Canada may have ratified a treaty and accepted international obligations, it might need to rely on the provinces to fully implement the treaty. Canada is nonetheless under an obligation to pursue the implementation in good faith, and is liable under the treaty.
An example of this on display was a settlement Canada paid to AbitibiBowater, a forestry company, due to actions taken by the provincial legislature of Newfoundland and Labrador in violation of NAFTA.
Yes, treaties can be indefinitely delayed
For example, the United States never ratified the Treaty of Versailles that ended the First World War against Germany. This is only one of many. Of course, because the legislature is inherently involved in the ratification of treaties in the United States, this may not qualify as a dualist entity.
For a more directly dualist approach, the Section 51(xxix) of the Australian Constitution gives the Commonwealth Parliament the right to legislate on "external affairs" which includes the implementation of international treaties into domestic law. Australia has agreed to be bound by human rights treaties but they are only enforcable once incorporated into domestic law - some parts of these treaties have not been.
In general, no country is a pure dualist.